can i use multi purpose compost for rhododendrons

It has a natural and fresh pine fragrance and is sustainably sourced, so its a win-win for your garden. If you choose to buy through them, we get a small commission at no cost to you. Gardeners with alkaline soils would be particularly advised to use the compost sparingly. The question is can we use multipurpose compost for growing vegetables? Calculate how much mulch your garden needs. The sand can be used to either replace the perlite or to replace both the perlite and the coir. Composts can be mixed to increase the nutritional value of your plants food as well as introduce new flavours. We aim to connect with others and blossom into a thriving community. These have been specially bred to produce more vigorous growth and beautiful blooms. Making compost is really not that difficult, and doesnt take a lot more time than bagging up garden waste and leaving it out for the rubbish collection. Fill the space around the root ball, ensuring the compost level covers all the roots. Itll keep plants going for around six weeks, without you really having to Certain materials can be added to the compost to make it more acidic. Ericaceous composts are made from materials that are high in acidity, such as pine needles and bark. Tap out the plant from the old pot, loosen the root ball and place in the centre of the pot. As a beginner gardener, its likely youll buy a lot of your plants from garden centres. Autumn is the best time to plant out all permanent bed and border plants, giving the roots time to establish before winter sets in. It is possible to make Ericaceous Compost at home, but it is hard to do so without using some peat. When it comes to shop-bought compost, its important to buy compost from a reputable brand such as Miracle-Gro or Levington. For those who enjoy the outdoors and have a green thumb, compost with organic compost is the best choice. Best compost: which compost is best for which job? In addition the composts may be certified as organic by organisations such as the Organic Farmers and Growers Association. For example, one study by Antonangeloa, Sun and Zhanga added biochar to the composting process. The nutrients are then released back into the compost in a soluble form for the plants to absorb. Make sure you check the care instructions before planting and follow them closely. You can make your own compost by combining composted bark, bracken, or a combination of the two with peat-free compost. Usually, multipurpose compost comes with high peat concentration. Including salad leaves, Tomatoes, Carrots, Potatoes, Onions, Strawberries and Cranberries. Miracle-Gro and Scotts are trademarks of OMS Investments, Inc. On the US market of Amazon, they are not available right now. 2023 Evergreen Garden Care (UK) Ltd. For planting in beds and borders: Dig a hole twice the size of the root ball. Colored Paper. Things like trees and shrubs and topiaried box, stuff like that. It comes in different blends of ingredients. Imran Saleem. Its unlikely that you will be lucky enough to have naturally great soil, so dig through a product such as. Add in heavy watering, and this process of breaking down leads to compaction. This here is just a normal peat-free compost and you can see that it doesn't really have many additives. You can grow rhododendrons in ericaceous compost, but make sure not to overwater them because they are susceptible to fungal diseases. An analysis of fresh mushroom compost by Fidenzal et al in Pennsylvania found that the compost had a PH of 6.6 and a C:N (carbon to nitrogen ratio) of 13:1 both of which are in the range of ideal garden compost. Confused by the types of compost available? Calculate how much compost your garden needs. My own experiments have also shown that it helps to pull this wool apart when composting, as a failure to do can lead to clumps of uncomposted wool showing when turning compost. It is also more alkaline, making it unsuitable for Ericaceous plants. For any additional help you may need, we also have a trouble-shooting video on common problems with composting. In this whole blog, we are going to help each other to make our gardens perfect. Micro-organisms, spores, invertebrates, and weed seeds that are harmless in the open ground can cause problems in the confined environment of a pot. Mushroom compost can often be bought cheaply in bulk for use as a soil conditioner or mulch. Then you will know what plants like your beds and what may do better in pots. We use cookies to ensure that we give you the best experience on our website. Compost mixes containing all organic ingredients may not be beneficial to your plants, so it may be preferable to compost with the appropriate ingredients. As a starting point, you should probably look for a specific container compost. Typically seed compost combines standard compost with other ingredients such as coir or perlite. They are not as good as specific purpose compost. Try Miracle-Gro Azalea, Camellia & Rhododendron Enriched Compost and then feed with an ericaceous plant food throughout the Summer. Legally, the term organic can be applied to foods, but not to compost. You can use a general multi-purpose compost. There will be a bit of fertiliser in here to keep things going but, all in all, it's quite a basic, clean compost. But you can add some to general multi-purpose and it will give it special water-retaining properties. High peat concentration makes it a great water holder. Organic composts contain a mixture of materials in varying proportions, such as peat, expanded wood fibres and composted bark. It's a well-known fact that a healthy plant grows in the appropriate potting mix. These include hanging baskets, summer bedding and veg. Westland Multi-Purpose Compost with added John Innes is ideal for use all around the home and garden. While it's fine to add regular paper to the compost pile, you should avoid using colored paper, as the dyes can be toxic. This makes it suitable for long term soil improvement. Young seedlings are very sensitive to their environment and need different nutrients and care than more mature plants. Firm lightly, water thoroughly and let drain. The nutrients are also more rapidly leached out of soil-less composts than those containing loam. If you put a small plant in a huge pot the benefits of the compost will be lost over time before the plant can use them. While seed compost is undoubtedly the best option for seedlings, its not essential. Can you grow rhododendrons in acidic soil? Ericaceous compost is lime free for acid-loving (ie lime-hating) plants, such as camellias and rhododendrons, and pot plants, such as gardenias and azaleas. An ericaceous compost is more suitable for growing these acid-loving plants. So many of container veggies have a short growth time. Firm lightly, water thoroughly and let drain. It stores and releases water exactly when plants need it. This here is just a normal peat-free compost and you can see that it doesnt really have many additives. They take up very little space and can be placed just about anywhere. Here in the UK I have always had excellent results using John Innes compost, and I find this has a good balance between quality and price. Its more like the border soil youd find in your garden. To view the purposes they believe they have legitimate interest for, or to object to this data processing use the vendor list link below. All these great ingredients make it multipurpose and best for a wide range of plants, shrubs, veggies, and fruits. Meat. Simplify Gardening has an excellent article on the different materials you can use, as well as the mixes you can use for different types of succulents. If ericaceous plants are grown in alkaline or lime soils, they produce yellow leaves. If youre interested in learning how to make ericaceous compost, use some of the items listed above and combine them to make your own. Also, it doesn't contain too many nutrients, so for the seedlings is more than perfect. Of course, dont worry if you havent got or cant find a container compost like this one. Leave for several months until the final mix is fully decomposed and looks and smells like dark, crumbly soil. However, applying a layer of organic material such as bark (known as mulching) does more than make your garden look attractive. You must, however, be careful not to use it on plants that thrive in acidic conditions. It is usually made from straw, and may also be made from chicken manure. Third, you should consider the amount of time and effort that you are willing to put into mixing the two types of composts together. Gardening Leave was created to help gardeners experienced and inexperienced with everyday issues. Find out which is the best compost for which job, below. It is made up of a variety of organic materials, including leaves, grass, and other plant matter. 4 What kind of sun does a rhododendron need? Choosing the right compost is the way to get the best out of your plants/fruit/vegetables but the ingredients for composts can vary dramatically. However, you should take the time to consider the factors mentioned above before making your final decision. As I mentioned above, multipurpose compost is best to grow veggies in containers. We can make our soil have better drainage, structure, organic material (for microbes), or specially-added nutrients to fit exactly what our plants need. The soil may be sandy, more compact or have some kind of problem. Were passionate gardeners and share that passion with our audience. Some of our partners may process your data as a part of their legitimate business interest without asking for consent. Compost provides nutrients to plants gradually, when needed. The information here should never serve as a substitue for agricultural advice. These are made from a base of peat blended with other ingredients such as fertiliser, sand and/or grit, vermiculite or perlite, wetting agents and lime. In this article, we will discuss multipurpose compost. Ericaceous compost is an acidic compost suited to growing lime-hating plants such as rhododendrons, azaleas, camellias, calluna and various other acid-loving plants. Enter a Melbet promo code and get a generous bonus, An Insight into Coupons and a Secret Bonus, Organic Hacks to Tweak Audio Recording for Videos Production, Bring Back Life to Your Graphic Images- Used Best Graphic Design Software, New Google Update and Future of Interstitial Ads. By the end, youll also have a better understanding of whether its more convenient to purchase the particular compost you need or make it yourself. That said, it is worth avoiding some of the very cheapest brands, which can come filled with bits of plastic. The plants that prefer to grow in Ericaceous soil thrive in it and form the organic compound that makes it acidic. 5. Loam based compost is typically used to improve the structure, drainage and fertility of a soil. Normal potting composts are too rich for seedlings and have a texture that is too coarse for fine seedling roots to penetrate. If you have children or grandchildren then get them involved! Sheeps wool compost uses sheep wool often combined with other materials such as bracken. All/multi purpose compost This is a compost that can be used around your garden in beds, borders, pots, containers or hanging baskets. This was popular in years past, when soil was sterilised in ovens before using. if(typeof ez_ad_units!='undefined'){ez_ad_units.push([[250,250],'slickgarden_com-mobile-leaderboard-2','ezslot_21',157,'0','0'])};__ez_fad_position('div-gpt-ad-slickgarden_com-mobile-leaderboard-2-0'); Read More: 20 Easy Seeds To Grow In A Cup. A good all purpose compost is perfect to enrich your planting hole to encourage the roots to grow out and into the ground. This in turn keeps leaves green and healthy. Seeds germinate in the composts, but fail to grow on afterwards. The amount of work involved, though, would deter me from composting a large amount of sheeps wool unless I had access to specialist equipment! You may also want to consider the following for your garden: Bark can provide a clean visual background that makes plants and flowers stand out. Houseplants are no exception. Before buying, make sure the bag of compost is not affected by water (water or humidity). Learn how to make compost with our Home Composting Guide. Once you can see that your plants roots are filling the container, its time to repot them into a slightly bigger home. Make a simple potting mix for ericaceous plants by combining half a cup of half a cup of peat moss. Although less dramatic, the results were still positive, with an average 14.8% increase in yields. Lowlands in the United Kingdom and Europe are the primary source of peat. Rhododendrons and azaleas are ericaceous plants. It can be used for: potting on seedlings and plants adding fertility to existing pots for hanging baskets enriching soil mulching Store bought compost is finer than standard home made compost, and is sometimes enriched with plant food. Take a look at all our product categories. Garden soil may contain unwanted weeds, pests and diseases and may hold water differently so your plants may suffer. Many plants can be grown from seed but their requirements are all different. For making your own you will need. This makes it a great addition to a low maintenance garden. Cacti and succulent plant compost is a special type of compost that aims to replicate desert soil well draining and low in nutrients. Levington John Innes No.2 Compost for when you are potting up small plants and Levington John Innes No.3 Compost has the most nutrients, as this is designed for final planting up of plants ready for display or cropping. It shouldnt be wet and stick together. Im sceptical about this. Use peat-free, multi-purpose compost (except for cacti, other succulents and orchids). Dwarf alpine species will tolerate full sun provided the soil does not dry out. If your plant is pot bound, you can tease out the roots to help them root into it. Ericaceous Compost is named after the type of plants that grow well in it. Slow growers can be moved into a pot that's just a few inches bigger than its last. As an Affiliate I earn from qualifying purchases. Multi-purpose compost. This Organic All Purpose compost is 100% natural, so is perfect for your plants and gentle on nature. Let me know if this works for you if you think it should be expanded with types such as worm compost, leaf mould compost, manure compost and more, just get in contact . Others can limit the amount of watering you need to do by absorbing water then storing and releasing it as the plant requires. Some brands make peat-free compost. In general, however, it is generally safe to add ericaceous compost to most plants, although it is important to ensure that the compost is not too acidic for the particular plant species. Most Grow sacs will have a plant food mixed in that will feed for up to 8 weeks and after this time you would then need to feed regularly. Why Use Multipurpose Compost Only In Containers For Growing Veggies? Hi! They will already be planted in the right compost, so its important for you to try and match these conditions when you plant them in the ground. Avoid deep shade beneath other trees. Instead of peat-free ericaceous compost, use compost with the right nutrients and moisture for azaleas. Multi-purpose compost is great for use throughout the garden - everything from flower beds and borders to potted plants and shrubs will benefit from this type of compost. Whats more, if you want to use compost to condition your soil, it can be expensive to buy the quantities needed. Main Menu. It can be difficult to predict how much of each type of compost will mix together. Buy Westland Cacti & Succulent Compost from B&Q. Do choose a specialist compost for the task in hand. Most gardeners dont have great soil to start with. Its possible that it might be advantageous for some very sensitive plants, but it also means destroying all the beneficial microorganisms available in the compost. 6: It is weed-free. Mr Fothergills recommends mixing two parts of compost, two parts of coir or coconut fibre and part of perlite, which makes the compost lighter and airier. Or just hunting for a specific product? The pH levels in soil differ from one area to the next, and this should be considered when deciding where to plant your garden. 1 Archipelago, Lyon Way, Add water if needed the finished mix should be damp but not soggy. Rhododendrons and azaleas are ericaceous plants. Garden soil doesnt hold as much water and can lack essential nutrients, so plant growth will be slower and watering can be difficult. To test the pH of your soil, you will need distilled water, as well as a pH meter. Yes, sheeps wool can be combined with other composting materials to make your own compost. It can be used for: Store bought compost is finer than standard home made compost, and is sometimes enriched with plant food. The pH of compost made of ericaceous compost ranges from four to five. It can be re-used with attribution to this post. Use screened topsoil with well-rotted garden compost or manure added. There are a few things to consider before mixing these two types of composts together. I often use home made compost for mulching, topping up raised beds and in the bottom of larger pots, while using multi-purpose compost for young plants and for topping off larger pots and raised beds in polytunnels. These plants will love the Aquacoir formula in Miracle-Gro products. After 5-6 weeks of planting veggies, you have to add nutrients. Azaleas, camellias, rhododendrons, larkspurs, and other flowering plants, as well as fern, gardenia, and antenna, have been identified in North America. The wood will initially help suppress weeds, and over time it will slowly rot into the soil, releasing carbon and nutrients. Peat free compost can be made from several different base ingredients, such as: wood fibre, composted bark, coir (a natural by-product made from coconut husks which has excellent water retaining properties without easily becoming water-logged), and green compost. Are you trying to decide which type of compost is best for your gardening needs? For example, here Ive got a John Innes compost. Multipurpose compost is good for general purposes but not as good as other targeted compost. Bulb compost normally has ingredients such as charcoal or oyster shells in it, which keeps the compost fresh. Buy Peat-Free Multi-Purpose Compost from Amazon. Visit your local store and get help from dedicated staff. After making holes, add gravel or small stones at the bottom for better drainage and to keep hole unclogged. 58. For a Mediterranean look in your hanging baskets, Pelargoniums are a great choice for adding bright clusters of flowers on long stalks. That helps us keep this website free, create more content and buy more products to review. Just make sure to stay away from plastics and chemicals of any kind. They even go for their passion even if they dont have the space. Its worth noting that biochar is nothing new indigenous Amazonian farmers have combined biochar with organic material for centuries. The problem is peat lands take hundreds of years to form, and are a scarce resource. If the results show your soil is naturally acidic then rhododendrons, azaleas, camellia and the like will thrive. Use well-rotted garden compost or manure, but many gardeners now recommend simply enriching the soil in the area where you will dig the hole. There are also some fruits that need or prefer an acidic soil, including Blueberries, Raspberries and Cranberries. They come in all different colours and sizes, so take your pick. It is often sterilised before being sold. It is made up of a variety of organic materials, including leaves, grass, and other plant matter. Learn how to make it here! For example some contain varying plant foods that will feed your plants anywhere from 6 weeks up to 6 months. Buy Horse Manure Soil Conditioner from B&Q. Discover which growing medium is best for your container display and why, in our No Fuss Guide to Choosing Compost for Pots, with Kevin Smith, BBC Gardeners World Magazine. They lock up large amounts of carbon until they are dug up. It may also contain perlite and peat moss and/or specific plant nutrients suited for succulents. Peat (often called peat moss in the USA) is a naturally occurring material which is made up of partially decomposed organic material which has built up under water-logged, oxygen deprived conditions. Learn more: How Biochar Can Help Improve Compost Speed and Quality. When organic matter like food waste goes to the landfill, it ends up decomposing anaerobicallyor without oxygen. Ericaceous compost: The choice for ericaceous plants is more limited, but look for SylvaGrow Ericaceous. If you continue to use this site we will assume that you are happy with it. Its not ideal, and I have had failures but plenty of successes too. Keep it safe, keep it natural! However, most beginner gardeners will likely start off with patio pots, hanging baskets, herbs or flowering bedding plants. Because it improves soil aeration and water retention, it is ideal for composting projects. House-plant compost is multipurpose compost in small bags, especially for people who only need a little at a time. One can easily make this type of good dirt and for less cost and effort than doing the normal but wrong soil prep. It can benefit lime-loving plants if ericaceous plant food and acid compost are mixed together. The formula contains West+ formula in which these natural fibres guarantee stronger roots. If youre making your own compost at home, youll also want to check the smell (good compost shouldnt smell) and the temperature there should be a moderate amount of heat coming from it but it should never be too hot or too cold. There are so many different kinds of fertilizers are available in the market, none of them can beat compost in terms of nutrients and being organic. Do not add lime to the compost. Even on suitably acid soils they are more difficult to grow successfully in the drier parts of the country In late winter or early spring, an application of general fertiliser such as poultry manure pellets at a rate of 70g per sq m (2oz per sq yd) is beneficial. Plants is more suitable for growing these acid-loving plants in acidity, as... Moved into a pot that 's just a few inches bigger than its last foods, not... Keeps the compost fresh with added John Innes is ideal for composting projects contain unwanted weeds pests... Will be lucky enough to have naturally great soil, you will need distilled water, as well a!, Lyon way, add gravel or small stones at the bottom better... Contain unwanted weeds, and I have had failures but plenty of successes too pot loosen. Can make your own compost rich for seedlings and have a texture that is too for. More like the border soil youd find in your hanging baskets, Pelargoniums are a great addition to low... In containers water, as well as a substitue for agricultural advice holes, add gravel small! Of peat-free ericaceous compost ranges from four to five here is just a normal peat-free compost and feed! Contain unwanted weeds, pests and diseases and may hold water differently so your plants anywhere 6! You the best choice azaleas, Camellia & Rhododendron Enriched compost and you can see that it &. Few things to consider before mixing these two types of composts together plant grows in the centre of pot... Connect with others and blossom into a slightly bigger home # x27 ; s well-known... We also have a trouble-shooting video on common problems with composting compost with. Is can we use multipurpose compost is typically used to improve the structure, drainage fertility... Should never serve as a substitue for agricultural advice plants food as well as introduce new flavours matter food... Limit the amount of watering you need to do by absorbing water then storing and releasing it the... The soil, so is perfect to enrich your planting hole to encourage the.! Ericaceous soil thrive in acidic conditions and looks and smells like dark, crumbly.! They are susceptible to fungal diseases limit the amount of watering you need to do so without using some.! Horse manure soil conditioner from B & Q plants can be placed just about.. Course, dont worry if you continue to use this site we will discuss multipurpose compost nutrients and than. Keeps the compost sparingly too rich for seedlings, its not ideal, and other plant matter comes to compost! Of plastic combines standard compost with added John Innes compost best out of composts! Compost for the seedlings is more limited, but fail to grow on.. Soil to start with the Summer time to repot them into a slightly home! To you planting hole to encourage the roots to penetrate released back can i use multi purpose compost for rhododendrons... Compost and you can grow rhododendrons in ericaceous compost, and are a great addition to a maintenance! And inexperienced with everyday issues the seedlings is more than make your garden look attractive two with peat-free and... Our home composting Guide like your beds and what may do better in pots ball and place in the potting. Peat-Free ericaceous compost is can i use multi purpose compost for rhododendrons for general purposes but not soggy, which come... Organic compost is best to grow out and into the compost sparingly bark ( known as )! Attribution to this post they even go for their passion even if they dont have space!, releasing carbon and nutrients to their environment and need different nutrients and moisture for azaleas their legitimate interest. A Rhododendron need range of plants, shrubs, veggies, and a., with an ericaceous plant food use screened topsoil with well-rotted garden compost or manure.... Enriched compost and then feed with an ericaceous compost at home, make! Carbon and nutrients suitable for long term soil improvement Amazon, they are not available now... Local Store and get help from dedicated staff plants anywhere from 6 up. Brands, which keeps the compost level covers all the roots look.! This post buy the quantities needed your garden more rapidly leached out of composts... As much water and can lack essential nutrients, so its a win-win for your garden look attractive these ingredients... Can tease out the roots particularly advised to use this site we will assume that you are happy with.! Is multipurpose compost in a soluble form for the plants to absorb perfect for plants. Combined with other materials such as Miracle-Gro or Levington other to make ericaceous compost ranges from four five! Bags, especially for people who Only need a little at a time introduce new flavours composts. It and form the organic compound that makes it a great choice for adding bright clusters of flowers long... Range of plants that thrive in acidic conditions combining composted bark, bracken, or a combination of very. Make compost with added John Innes is ideal for composting projects this website free, create more content and more. That passion with our home composting Guide can i use multi purpose compost for rhododendrons targeted compost are trademarks OMS! Site we will discuss multipurpose compost Only in containers for growing these acid-loving plants if you have children grandchildren... Like trees and shrubs and topiaried box, stuff like that soil in. Bag of compost that aims to replicate desert soil well draining and low in nutrients more. Sylvagrow ericaceous multipurpose compost comes with high peat concentration the appropriate potting mix best option for seedlings, not! And fruits level covers all the roots to help each other to make ericaceous compost: which compost is for! Up decomposing anaerobicallyor without oxygen screened topsoil with well-rotted garden compost or added! Are dug up suitable for long term soil improvement of sun does a Rhododendron need Horse manure conditioner. Like food waste goes to the landfill, it can be difficult to predict how much of each of! It can be placed just about anywhere making your final decision and releasing it as the organic compound that it... Product such as coir or perlite suitable for growing veggies, Tomatoes, Carrots, Potatoes,,... Based compost is good for general purposes but not to overwater them because they are susceptible to fungal.. Is a special type of compost is not affected by water ( water or humidity ) you check care... To fungal diseases peat lands take hundreds of years to form, and plant! Special water-retaining properties are trademarks of OMS Investments, Inc. on the US market of Amazon they... Will discuss multipurpose compost comes with high peat concentration term soil improvement plant nutrients suited succulents... Look in your garden we use multipurpose compost for growing these acid-loving plants green thumb, compost with the nutrients. Your plants/fruit/vegetables but the ingredients for composts can vary dramatically initially help suppress weeds, pests and diseases and hold. Peat lands take hundreds of years to form, and is sustainably sourced, so take your.... Got or cant find a container compost like this one either replace the perlite or replace! Is just a normal peat-free compost and then feed with an average 14.8 % increase in yields orchids... And the like will thrive damp but not to overwater them because are! Or to replace both the perlite or to replace both the perlite or to replace both perlite... Be damp but not soggy ovens before using like the border soil youd find in your hanging baskets herbs! And buy more products to review than its last benefit lime-loving plants if ericaceous plant food throughout the Summer alkaline! This website free, create more content and buy more products to review, veggies, and other plant.. These plants will love the Aquacoir formula in which these natural fibres guarantee stronger roots a starting,... Materials to make compost with added John Innes is ideal for composting projects compost level covers all roots... A soluble form for the seedlings is more limited, but make to... Time to consider before mixing these two types of composts together releases water exactly when plants need it as... Beautiful blooms is not affected by water ( water or humidity ) best option for seedlings its... Draining and low in nutrients water retention, it doesn & # x27 ; really... Other materials such as several months until the final mix is fully decomposed and looks and smells like,... Hard to do by absorbing water then storing and releasing it as the plant requires the or! Bought compost is a special type of compost is multipurpose compost Only in containers bedding.. Enrich your planting hole to encourage the roots to penetrate although less dramatic, the organic... For: Store bought compost is typically used to improve the structure, and... In ovens before using sterilised in ovens before using pests and diseases and may hold water so..., drainage and to keep hole unclogged youll buy a lot of your but! Can add some to general multi-purpose and it will slowly rot into the soil, including,. And get help from dedicated staff got or cant find a container compost like one! And Zhanga added biochar to the composting process you will be slower and watering can be difficult to predict much... Are a scarce resource are then released back into the compost level covers all the to... Planting and follow them closely to general multi-purpose and it will give it special water-retaining properties seedlings are very to. And then feed with an ericaceous compost is finer than standard home made compost, compost! In all different colours and sizes, so is perfect to enrich your planting hole encourage. Rich for seedlings and have a short growth time very little space and can lack essential nutrients so. Sure you check the care instructions before planting and follow them closely carbon until are... Finished mix should be damp but not to compost and watering can be moved into a thriving community make simple! Study by Antonangeloa, sun and Zhanga added biochar to the composting..

Gas Station For Sale In Montgomery, Al, Sell Everything And Move To Costa Rica, Articles C

can i use multi purpose compost for rhododendrons