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Growing Rhododendrons – Care guide and growing tips
Last Updated on January 21, 2020 by John
Rhododendron can be very rewarding plants to have in your garden but it’s important to know how to plant them, how to maintain them, and how to prune them so that you can enjoy them long-term.
Its equally import to know what soil type and position they grow best in as this is essential for Rhododendrons. If planted in the ground they need acidic soil which is ph 5 or 6 and if grown in containers they need to be planted in ericaceous compost with John Innes ericaceous loam-based potting compost being the best mix.
Where to plant
When you start about planting rhododendrons you need to select a site that will be conducive to long-term growth and success for your rhododendrons. This should be a location in your garden that offers dappled shade and it’s somewhat sheltered from cold winds in winter. You don’t want deep shade, a location underneath other trees may be trial and error as sometimes they do well under trees and sometimes not. Most of these plants will tolerate open sites as long as they are sheltered from cold winter winds but we recommend avoiding planting them where they get the morning sun. If you have a dwarf variety they are likely more tolerant of full sunlight and grow well in pots making them perfect for patios and rockeries. The only thing you will need to keep a check on is that the soil does not dry out as they do not like dry soil, this could simply mean more watering over summer.
Type of soil
Rhododendrons require acidic soil that is rich in organic matter so you may need to test the soil ahead of time to make sure it’s somewhere between a pH of 5.0 and 6.0. They also require well-drained soil so make amendments as necessary, add some acidic organic matter such as composted tree bark or conifer clipping into the soil with grit to improve drainage. If your soil is tested ahead of time and it turns out to have higher levels of alkalinity you will find it very difficult to improve the soil enough to grow rhododendrons. If this is the case you can try growing your rhododendrons in a container planted in ericaceous compost.
Choosing the right type of rhododendron for the right place
If you have a shaded patio with containers, the smaller compact, hybrid options are going to be a much better solution as they don’t grow as big but are still larger than the smaller dwarf varieties. If you have a rockery and are looking to add some dwarf shrubs to the mix, the dwarf varieties will be better for you and will also handle full sun better. If you have a woodland garden, look for the larger rhododendron as it is there native habitat and they will grow very well there.
When to plant rhododendrons
When you are ready for planting, October is your first option which is when they typically go dormant. Conversely you can plant at the end of March or beginning of April. As rhododendrons are typically sold in pots, you can also plant them anytime in summer if needed but they will require watering frequently. Ideally, you want to be planting October or March/April
Preparing the ground with organic matter
You want to add plenty of acidic organic matter to the soil to give them the best start, think like composted pine needles or tree bark provide the best possible growing conditions for your rhododendron. When you do this you want to mix it well into the existing soil so it’s well mixed in. Do not mix in normal compost as this is not acidic if you mix compost into the soil or use it as a mulch ensure you use ericaceous compost.
When you plant your rhododendron remember that it is a surface rooting plant so the roots should be just barely covered. If you plant it too deeply it will not thrive and is an easy mistake to make. Once you have it planted you should apply a good layer of acidic mulch such as ericaceous compost or tree bark in a well-aerated fashion. Basically, you don’t want to pack it down but rather help it to keep the roots cool.
Growing rhododendrons in containers
If you are planting rhododendrons in containers, you want to pick a loam-based potting compost mixture, we found that John Innes ericaceous potting compost is perfect. It is very important to pay attention to the drainage and to make sure that your plant gets proper drainage and proper airflow. We recommended that you repot your rhododendron every few years using fresh potting compost, this something you should do at the beginning of Spring. In the years in between, you should simply replace the top few inches of compost.
In terms of your rhododendron care, feeding and watering rhododendrons are all something that should be done on a regular basis. Rhododendrons will thrive in areas that receive high amounts of rainfall so if you live in a dry part of the country it’s important that you water regularly.
Avoid using tap water if you live in a hard water area
Something to note is that if you live in an area with hard water (water companies will be able to tell you if you have hard water), you don’t want to use tap water regularly because hard tap water has too much calcium and it reduces the acidity in the soil and subsequently in the roots of the plant. We recommend using rainwater such as that collected in a water butt. However tap water can be used for a few months in summer if needed, it’s just not recommended for watering all year round.
One of the reasons that people notice yellowing leaves on the Rhododendron is because they use tap water which is too hard. If at all possible, collect rainwater throughout the year as mentioned and use that to water your rhododendrons. If that runs out you can always go to tap water for a month or two in the hotter parts of the year as mentioned.
In terms of feeding, you should add ericaceous fertiliser in the springtime on an annual basis. This will help to ensure the root structure gets all of the nutrients necessary. Every Spring when your soil is moist you can also renew the mulch around the base of rhododendrons.
When it comes to pruning rhododendrons you don’t have to do much other than removing the spent flowers which is known as deadheading and cutting our any dead or diseased wood. You can also prune to restrict the size of your rhododendrons if necessary but ideally, you want to choose a position that is big enough for the variety you plant
In either case, if you are deadheading most of the time it’s recommended that you use pruning secateurs or sharp scissors. With this process, you want to sterilise whatever tool you are using. This is particularly true if you are removing dead or diseased branches as failure to sterilise in between each cut could accidentally transfer things like fungus or bacteria to other parts of the plant or onto other plants.
If you have an established plant that is out of control or leggy, you can do a hard pruning where you cut it back one-third of older stems to the ground and then cut the remaining stems back by around a third. If you need to cut it back any more then we recommend cutting it back over two years. You want to do hard pruning between November and March. It will return the next spring with newer growth, refreshed and ready to bloom.
With rhododendrons, you have to be cognizant of different pests and diseases as well as problems that could take place. Rhododendrons typically produce flower buds at the end of summer but if you have a dry spell in your area at the end of summer the flower buds might not form properly or they could form only partially and then end up dropping off. You can combat this by mulching regularly as mentioned above and watering during the dryer times.
Wilting leaves and leaf drop
Regular rhododendrons the same as all evergreen shrubs will experience a small part of their foliage shedding during the Spring and Summer however leaf drop can also happen if you are experiencing drought in your area. If the Rhododendron roots become waterlogged it can also result in leaf drop. Obviously, if you noticed leaf drop you need to verify whether you’ve experienced a period of drought and if you haven’t or you don’t see the leaves browning at the edges, you may have problems with drainage which we previously talked about hear.
If the leaves are not falling off but they are drooping it could be the result of severe cold weather. If you are experiencing a cold snap on your leaves are drooping then they will return once the conditions improve and the weather warms up a little
Rhododendrons can be damaged by windy weather which can result in damage to the flowers or cause the leaves to get scorched by sunlight which is why it’s recommended that you plant them somewhere they are protected from cold winds.
Pests and diseases
Vine weevil and scale insects
Insects that you need to be aware of include the Vine Weevil which is particularly problematic for Rhododendrons grown in containers because not only do the adults eat the leaves, the baby grubs also eat the roots and when grown in pots this is when it can really become a problem.
Scale insects are also problematic as they suck the sap from the foliage causing damage to the leaves, the problem is they also excrete honeydew which is a sticky substance but this also encourages black sooty mounds.
Diseases such as Azalea Gall, Bud blast, and honey fungus are among the most prominent of issues.
Azealia Gall is a type of fungus that won’t kill your Rhododendron but it will disfigure it. It’s a form of airborne Spore that leaves small galls on the leaves that could range in size from that of a pea to a small plum. They start out pale green and turn slightly red at some points. Eventually, they become white. You can remove any infected leaves to prevent the spread. If it gets too severe you will have to replace your plant altogether. This is much more common on the indoor rhododendrons, the type you usually buy as house plants.
Honey fungus is a type of fungus that attacks and kills the roots. It spreads underground and starts to attack first the roots of the rhododendron and then the causes dead wood. This is actually one of the most destructive fungal diseases across the UK and no plant is immune. Given that it does attack underground first, you won’t see the dead or decaying roots but you might notice mushrooms or notice some white fungus material on the bark lower down. Most of the time the above-ground symptoms include smaller leaves, failing a flower, cracking at the bark, and death for your plant. Unfortunately, there are no chemical control measures available so you would have to evacuate the damaged plant and burn it or otherwise destroy it.
Rhododendrons can be a little difficult to propagate, you can take cuttings from the current season’s growth which we have a guide on here. Another way to propagate rhododendrons is by layering but this takes up to 3 years so takes a lot of patience. You can also sow seeds around January in a heated propagator.
Our favourite varieties
Rhododendrons come in three main types. The first are evergreen shrubs which are smaller than trees and typically produce flowers all year round. The second are deciduous shrubs which are much smaller and typically shed their leaves once in the Autumn and once again in the winter. Evergreen rhododendrons will grow into the shape of trees and remain green throughout the year.
There are over 1,000 species that fall into these three main categories but some of our favourites include the rhododendron Elviira. This is a broadleaf, evergreen shrub that produces cherry red flowers. The flowers are shaped like a trumpet and they have a beautiful scent to them that is not too overpowering. This one is very low maintenance as far as shrubs go and all you really have to do is deadhead.
Another favoured variety is the Rhododendron blue Peter which is pictured above. This is an evergreen shrub and it has a colourful, dense head on the leaves with violet-blue flowers that come in funnel-shaped, tubular shapes, or Bell shapes. They also offer frilled pedals which is very unique and quite stunning. This is a bushy shrub that will reach up to 3 meters in height. It is ideally suited to be a hedge, privacy screen or specimen plant.
Rhododendron Bow Bells which is pictured above is another favourite that is an evergreen shrub which will grow in a vertical fashion and reach approximately 1 m in the span of 10 years. So much slower growth compared to others. The flowers are shaped like a bell and they growing loose clusters taking on bright pink shades.
The Rhododendron Windsong stands out with its yellow, funnel-shaped flowers that include red nectarines in the centre. These flowers take on a bell shape and can actually hold 14 flowers in a single truss. The
Rhododendron white angel, as the name suggests, forms white flowers that grow out of a vase-shaped cone and are very long and stunning. Each flower reaches about 3cm in diameter and you can enjoy clusters of flowers throughout the springtime. Maintenance is very little, all you have to do is deadhead here and there and then watch as the shrub takes on its round shape