Care Advice

3. CARE ADVICE

Rhododendrons are generally trouble-free plants provided they are planted correctly in a suitable position and given appropriate feed and moisture. Learn how to look after your plants here, with details on feeding, watering, pruning and spacing.

      3.1 HOW TO CARE FOR RHODODENDRONS

The most important aspect of care, is to plant correctly in the first place. See 1.7 How to plant. If the plant is in the wrong place or planted badly, it will not thrive properly. Newly planted rhododendrons should receive a light fertilizer dressing in early March and again after flowering. Try Millais Ericaceous Slow Release Feed. They also require plenty of water from June to September, particularly for the first two seasons. Mulching around the rootball will conserve moisture and help winter insulation. Young plants may require extra protection especially for the first 2 seasons. See 2.4 Hardiness Ratings. Deadheading is certainly worth doing on young or sick plants, and so is formative pruning to encourage a nice bushy habit. As the plant develops in the following years, do review plant spacing to ensure that the border is not overplanted.


      3.2 WATERING AND DRAINAGE

Watering:

It is worth remembering that many rhododendrons originate from monsoon regions of the Himalayas and are used to having plenty of water during June. This is also the time when flower buds are initiated for the following spring, so any dryness will reduce the set of flower buds.

Summer rainfall in the Himalayas can exceed 190cm, compared to an annual British average of 75cm. Whilst this amount of irrigation is not necessary, rhododendrons are shallow rooted plants which means that when moisture is short, they do not compete well against thirsty plants with stronger roots like large trees, conifers and grasses.

Newly planted rhododendrons are especially vulnerable to drought and may need extra watering during their first summer. Fortunately dry rhododendrons are easy to spot with their droopy leaves and perhaps collapsing young shoots, and as long as they are watered promptly, they should recover quickly.

We normally advise that rain water is best, but tap water is better than no water! Please consider the benefits of using any natural water on your property or saving roof water in water butts or ponds. These biological waters are better for rhododendrons and save the use of expensive chlorinated tap water. In times of drought, grey household water is perfectly acceptable, and slightly soapy water may even deter the aphids!

When watering, a good soak of the root-ball once per week is far better than a daily light sprinkle. A good soak will really get into the soil and be conserved. It will also make roots search out the moisture as it wicks away, and so encourage better rooting. A light sprinkle will only dampen the soil surface, encourage weeds and be evaporated away.

 

For those with large numbers of plants, do remember that drip irrigation is highly efficient at getting water where it is needed, and is normally exempt from hosepipe bans. Moisture conservation is important to help rhododendrons in periods of dry weather, so choose planting positions which are not too dry, or baked by hot sun.

Drainage:

Whilst rhododendrons in the Himalayas receive high rainfall, they usually have exceptional drainage, with alpine species growing on screes and rocks with little soil to retain the rainfall. Some species do grow in much damper conditions, for instance R. thomsonii and viscosum (Swamp honeysuckle) will grow on river banks, but never in stagnant conditions. In cultivation, good drainage is critical to prevent phytophthora type root rots. Drainage can be improved by installing land drains and digging open ditches. Planting on mounds with the whole rootball above ground can be particularly effective on damp heavy soils where there is high rainfall.

Good drainage is especially important for yellow flowering rhododendrons, which are weaker growing and will not tolerate damp conditions.

If you have a plant with yellowing chlorotic leaves that is not thriving on damp soil, try lifting it in the winter to inspect the roots. A thin plate-like rootball on the surface, with no white roots, indicates that the soil is too wet for the plant to survive.


      3.3 FEEDING

Just like us, rhododendrons do need plenty of water and food! So many people forget to fertilize their plants and this is particularly important in the first years whilst the plants are establishing themselves. It is also essential if you are growing your plant in a pot. Our range of INKARHO plants for limey soils are strong hungry feeders which need more feed than usual, otherwise they will run out of feed and turn yellow. Feed them and they will respond!

The main feeder roots of a rhododendron are near the soil surface and at the edge of the rootball. For this reason, when you are planting your rhododendron there is no point in putting fertilizer into the bottom of the hole. We recommend that you sprinkle the slow release fertilizer around the top of the plant, avoiding the stem, and gently hoe into the soil. Give a nice drink at this point too.

A good rhododendron fertilizer such as Millais Ericaceous Slow Release Feed applied annually in March and again after flowering in June is recommended to feed the plant all season. This is made to our recipe and has higher levels of iron, magnesium and manganese than most feeds, and will typically keep your plant in top condition. As a guide, use a teaspoon per 3 litre plant, a tablespoon per 5 -7 litre plant, and a small handful on a plant 1 metre tall, and 2 handfuls on a 2 metre tall plant. Fertilizers such as Gromore and pelleted chicken manure can be too rich and fast acting for rhododendrons, and we do not recommend bonemeal which contains high levels of calcium.

Liquid feeds are faster acting and are really useful to help sick plants green up more quickly. By watering or spraying onto foliage, the leaves make use of the fertilizer straight away. Maxicrop with Iron is our favoured liquid feed, as we have seen signs of scorch on sensitive varieties with other brands of liquid feeds.

Do not be tempted to over-fertilize. Some dwarf varieties and many choice species (e.g. taliensia and neriiflora) are particularly salt-sensitive and susceptible to over-feeding. Overfeeding shows as leaf tip burn where the edge of the leaf goes dry and brown. Do not use general feeds after midsummer, as this will encourage leaf growth at the expense of flowers next season! This new soft growth is then vulnerable to early autumn frosts. Avoid chicken and farmyard manures as they are too rich in nitrogen for rhododendrons and will cause leaf burn.

Towards the end of the summer, an application of Superphosphate is useful to help flowering, rooting, and also to slow down growth and toughen up the plant ready for winter. If you use our recommended feed it is unlikely that you need to add anything else, but we also sell ‘straights’ fertilizers to correct any deficiencies. See 4.1.2 Deficiencies for further information.


      3.4 MULCHING

Mulching every few years during the winter months is particularly beneficial to rhododendrons. Use organic matter such as leaf-mould, garden compost, bracken peat, pine needles, composted wood-chip, bark, or very well-rotted manure, and apply as a 3cm deep layer which will act as a sponge and retain water on dry soils during summer months. However, rhododendrons hate being planted to deep, so avoid mulching around the stem. Poultry and horse manure should be avoided unless black, and at least 3 years old. Do not use spent mushroom compost as this contains far too much lime.

Your plants will really love a good mulch and absorb its nutrients. Biological and worm activity will break down the mulch and improve soil fertility. Over the next few years, new roots will grow into this nice fibrous layer, and feed the plant. Recent research shows that as leaf-mould breaks down, manganese is released, and this is particularly beneficial to rhododendrons on neutral soils as it unlocks other nutrients and makes them available to the plants. Mulching has the added benefit of making your garden look attractive.

We do not recommend weed suppressing fabrics or matting. Any fabric will interfere with soil aeration, drainage and worm activity, and will prevent rhododendrons from growing new roots at the surface.


      3.5 DEADHEADING

Dead heading is the process of snapping off the spent flower trusses after flowering. It really does make a difference, and channels plant energy into strong new shoots with more flower buds instead of wasting loads of energy making unnecessary big fat seedpods. A plant that has been deadheaded straight after flowering may grow 4-5 bushy new shoots which can support more flowers next season, compared to 1-2 spindly shoots if the seed heads are left to remain and swell. We also believe that it prevents disease spreading from the rotting flower head down into the stems. Deadheading prevents brown seed heads developing and gives the plant that well-groomed look for the rest of the season.

On the nursery, we try to deadhead everything in early June, as we are convinced of the benefits. However, in a garden it is not always possible due to the size of the plants or the size of the garden. Therefore prioritize the operation, and concentrate on your younger plants, and those which seem weak or sick.

Simply snap the flower truss off at the base, above the whorl of leaves, using your thumb and forefinger. Be careful not to break off the new growth coming through underneath. You will soon be an expert and it will certainly pay off next season!


      3.6 PRUNING

As a young plant grows in the garden, it takes up considerably more space and height. Hopefully this is what is required, but at some point, it may become necessary to trim or prune your plants into shape. This can be achieved by pinching new shoots as they emerge in May and June, pruning with secateurs, or heavier pruning with loppers and saws. Our rule of thumb is that if you can cut a branch with secateurs then do so straight after flowering, but if significant pruning and a saw is required, then do so between Christmas and March.

Bud rolling and pinching:

On the nursery, we want to produce as bushy a plant as possible. We do this by ‘rolling out’ the terminal growth buds as they soften and start to swell in spring. This encourages the buds from the surrounding whorl of leaves to break into growth, so that instead of one new shoot, the plant grows multiple new shoots, which helps to create a dense bushy plant with more flowering next season. If bud-rolling is missed in a busy spring, you can do formative pinching-out using thumb and forefinger, or prune with secateurs in May and June. Simply take out the single central growth bud at each whorl, and instead of one lanky shoot, you should gain 3-4 bushy shoots each of which can support flowers the next season. If there are already several shoots above the whorl, then leave them all alone!

Light pruning:

Any light pruning that you can do with a pair of secateurs is normally best done straight after flowering. This way you can enjoy the flowers, and then prune away so that new growth starts where you want it to. Flower buds for the following season are initiated at the end of June and early July, so if you prune early enough, you should still have time for shoots to grow and for flower buds to form for the next season. When pruning back, try to leave some leaves on the shoot, as you will get better branching from the buds near the leaves. Azaleas can often grow long branches that may look a little unsightly after flowering. Simply snip these taller branches back to a natural plateau so that the plant reforms into a nice shape. Regular light pruning after flowering can dramatically improve the habit of a misshapen plant over the course of a few seasons. It is also recommended to control rhododendrons which are taking up too much space next to a drive or a path.


      3.7 HEAVY PRUNING AND COPPICING

What about pruning large established rhododendrons? Perhaps you have a plant that is out of control and creeping across a drive, or simply looking elderly and past its best. Sometimes radical action and heavy pruning is required, but there are no half measures and it will open up a large hole in the landscape, so give it some careful thought first. Once you cut into the foliage by more than about 30-40cm, there is often no foliage left, just bare branches which will not regenerate with the vigour required. This may mean that you are left with a leggy plant with no foliage at the base. The only real answer is to coppice back all the growth, leaving stumps between 50 and 150cm tall. If you can manage to leave a few green shoots, this will help to draw up the sap, and sometimes you can spread the work over 2 years by doing one side each year. The best time to carry out heavy pruning is in February-March, just before the sap rises in the spring.

New growth starts from dormant buds below the cut, forcing their way through the old trunks in April and May. This spring period is a critical time to ensure your plant is well fed, mulched, and given plenty of water. Over the next couple of years a dense and vigorous new bush will be formed from ground level. In fact, growth can be so vigorous that the young shoots can’t support themselves, so be prepared to trim them in the first season. After 2 years you should have a dense bushy plant without any old stems visible, and precocious flowering. Most rhododendrons do regenerate well, but weak growing plants, in too much shade, and those with smooth bark may fail to re-grow. You have been warned!

We have successfully rejuvenated very old specimens at Crosswater Farm Gardens, particularly after wet snow damage. We have successfully cut back rhododendrons that were well over 4m tall and fifty years old to stumps about 60-90cm above the ground. A good woodchipper makes light work of all the branches, and the chips can be fired back on the ground as a useful mulch.


      3.8 SPACING AND MOVING

Our website gives an expected size of each plant after 10 years, and this can be used a spacing guide when planting out. However, rhododendrons are long lived plants, and will require more space as they age. Often, it is best to plant reasonably close, to give a good effect in the garden after just a few years, but to space them further apart before they get too big. This gives a chance to review any plants which are not performing, move any clashing colours, and respace accordingly.

There’s an old saying ‘If you can move a rhododendron, you can move a rhododendron’. Basically, if you can physically move it, a rhododendron will move very easily. With the aid of machinery, we have moved plants 2.5m high and 4m wide. With smaller plants, a tarpaulin can be ruffled half way under one side of its roots, and then the plant can be tipped onto the tarpaulin to enable the leading edge to be pulled out. It can then be dragged to its new position before reversing the procedure.

The best time to move a plant is in the early autumn, when the ground is still warm and not too wet. If ground conditions allow, planting is possible up until the end of March. The rootball is not deep, rarely deeper than the depth of a spade on most soils. It is however wide and should emerge in the shape of a wide plate. As a guide, a plant 1 metre tall, will have a rootball about 75cm wide, and if 2 metres tall it will be about 1.2m wide. Do not be tempted to cut too close to the stem or the plant will not move properly. Start by cutting a circle around the stem with a spade, but if you meet resistance from roots, you will need to make a bigger circle. Scrape off any loose soil from the top of the rootball to reduce weight, and then lever the rootball gently up, making sure that it does not break up. At this point, you may be able to reduce the width of the rootball if the edges are loose.

Take the plant to its new position and dig a matching hole, plus 15cm on each side. Mix in some good compost at the base, and place the plant in the hole, ensuring that the levels are correct. See 1.7 Backfill the edges of the hole with a mix of compost and soil. Fertilize and water well for its first season.