A recent diary entry was entitled Rhododendron Choices for Small Gardens. In this I gave little information on cultivation and propagation, but promised to write a sequel rectifying that omission. Here it is.
Dwarf rhododendrons are easy to grow in any well-drained soil that is on the acid side of neutral (pH below 7). Like most members of the heather family (Ericaceae), they are intolerant of freely available lime. The chief symptom of intolerance is general yellowing of the leaves, particularly between the leaf veins. This is caused by the free lime preventing the plants from accumulating key elements for the manufacture of chlorophyll. The deficiency leads to reduced photosynthesis and hence suboptimal growth and flowering. If the situation is not corrected, the slow death of affected plants is likely to follow..
It is possible to grow dwarf rhododendrons on limy soils but this entails regular treatments with fertilisers containing chelated (bound) minerals including iron, manganese and magnesium which the plants can accumulate. Copious periodic additions of mulches comprising organic materials such as leafmould, composted wood chip and chopped straw can also help. These materials produce acidic humus that counteracts soil alkalinity.
An alternative is to replace all the soil to rooting depth with non-alkaline material, again containing ample organic matter. But the problem can be avoided altogether by growing small species and hybrids in containers. These should be large enough to contain the plant(s) with ease, allowing for future growth.
Rhododendron ‘Yaku Fairy’
All rhododendrons have shallow, fine, fibrous root systems that easily dry out. They are not adapted to water shortage and will die, or be severely restricted in their growth and flowering, if the soil dries out appreciably. Try to ensure that it always remains moist. Equally, most dwarf rhododendrons hate heavy, poorly drained soils subject to waterlogging. This all ties in with the soil conditions that they have mostly evolved to deal with as plants of open mountainsides and moorlands, rocky screes, and woodland edge habitats.
Whether in the open ground or pots, if your tap water is alkaline you will need to water with rainwater or deionised water. Otherwise the plants will eventually succumb to lime-induced chlorosis.
As indicated above, most dwarf rhododendrons, unlike many of their larger brethren, are plants of open habitats. Consequently, they grow and flower best in open or lightly shaded positions in the garden. Deeper shade will not kill them, indeed they may well prosper as far as vegetative growth is concerned, but flower production will be severely reduced. So any suitable spot in the open, with well drained but moisture retentive soil, will suit them best.
I have already mentioned the benefits of mulching, which apply to all rhododendrons, not just the dwarfs. I generally prepare a mix of about equal parts of almost dry, fine sieved leafmould, composted forest bark, and silver sand. Where possible I apply the mulch over the root system, but for the prostrate and ground-hugging types I spread it over the foliage, working it in well with the hands. Then I water thoroughly to ‘clean’ the excess from the leaves and ensure good contact of the mulch with the roots below.
The response in terms of the amount and health of new growth is reward enough for this small amount of effort.
Because of their shallow fibrous-rooted character, it is easy to move dwarf rhododendrons. The best time is in the spring or autumn when the roots are growing and air temperatures are not too high. But you can move them in summer providing they are never allowed to dry out. It is wise to enrich the soil around the roots of the transplanted rhododendron with some ericaceous compost. And, of course, pay careful attention to watering until the plant is well established.
Dwarf rhododendrons are not generally subject to serious pests. Plants grown in containers, particularly under glass, may become infested with aphids. Often, just moving the pots outside will solve the problem, but if not, the plants are usually small enough to enable the aphids to be removed without resorting to chemicals. In serious cases, power spraying with water containing ‘soft soap’ or similar will do the trick.
Vine weevils, both as adult beetles and (more seriously) as grubs, can cause problems. Adults generally nibble the edges of the leaves producing a fretwork pattern that is easily recognised and very irritating to behold! A search with a torch in the evening can be a successful means of keeping the numbers down.
The effects of grub infestation in the soil or potting compost are less easy to spot unless the numbers are large. Generally the nibbling away of roots leads to an unhealthy looking plant. Therefore the root system of any dwarf rhododendron that is failing to thrive should be examined carefully. Any crescent shaped, creamy white grubs should be removed and despatched. If the infestation is severe you may need to resort to the use of approved chemicals, applied carefully as directed. But I have never had to use such extreme measures.
Although there are many plant diseases that can afflict rhododendrons, I have only experienced three potentially serious ones. Honey fungus, Phytophora root rot (suspected not proven) and Rhododendron powdery mildew. I will deal with each of these diseases in turn, but it should be borne in mind that they are rarely of serious concern to the gardener.
Honey fungus is a serious disease of a wide range of trees and shrubs around the world, including the British Isles. It can lead to devastating financial losses in commercial forestry, and serious losses in collections of ornamentals, including rhododendrons. It is particularly prevalent in old gardens with over-mature trees and shrubs. But perhaps the chief source of infection is the decaying root systems of trees. These may have been felled either because they were infected, or for some other reason. The fungus feeds on the decaying stump, sending out bootlace-like composite hyphae (so-called rhizomorphs). These attach to and infect the roots of other plants. Infected root systems are starved of water and nutrients, with predictable results. Wilting is usually the first and most obvious symptom of infection, followed by rapid decline and (usually) death.
The best control method is basic hygiene. When moving or removing trees or shrubs anywhere in the garden, whether infected or not, make sure to remove as much as possible of the old root system. The worst thing is to cut trees down and leave the root systems in place. If you have an infestation and do not wish to control it with toxic chemicals, letting the ground lie fallow for 2-3 years may help.
Be aware that Honey fungus can attack herbaceous plants and bulbs as well as woody species. I lost large numbers of daffodils planted in ignorance in Honey fungus infested soil.
Root rots are a complex mix of soil-borne fungal and bacterial diseases that can work together or alone to undermine the health of your dwarf rhododendrons. Phytophthora root infections can be due to one or more of several species. The symptoms, however, are generally much the same, wilting followed by general decline. Death of whole branches follows that may lead to loss of the whole plant.
Root rots are usually restricted to plants growing in soils prone to waterlogging. So make sure that your rhododendron beds are well drained and do not stand wet after prolonged heavy rain. Dank, heavily shaded areas also favour these diseases, avoid planting dwarf rhododendrons in such areas. Effective chemicals are only available to professional growers, but should not be necessary if you get the cultivation conditions right.
A number of fungi in the genus Erysiphe can cause Powdery mildew diseases in rhododendrons. But for our purposes they all have much the same effects, so may be treated as one disease. As the name implies, the fungus generally produces masses of powdery spores on the leaves, particularly on the lower surfaces. But these are not always very apparent in infected rhododendrons. A more reliable symptom is yellowish brown leaf spots which darken as they mature to near black. Flowers may also be affected causing distortion of the petals.
Severely infected plans may loose some or all of their leaves, resulting in inhibition of growth and flowering. Infection is worse in some years than other, probably mainly due to varying climatic factors.
Some rhododendron species and varieties are much more susceptible to Powdery mildew than others. In our garden few of the dwarfs get it, chiefly those with red flowers having R. forrestii in their ancestry, including R. forrestii itself (a difficult plant to grow and flower well anyway), and the hybrids R. ‘Carmen’, R. ‘Scarlet Wonder’, and others.
The easiest means of control is not to grow susceptible species and varieties. If, however, you decide to take the risk, plant in good, well-drained soil in an open situation. Look out for leaf spots and signs of general ill health. If detected, remove the affected leaves immediately and destroy them. There are fungal sprays that may give some relief, but the infection is likely to return the following growing season.
Dwarf rhododendrons may be propagated by seed, and vegetatively by cuttings, layering and grafts.
The best method for increasing species, particularly if you want more than a few plants, is to raise them from seed. Some of the larger, more popular rhododendron species may be available commercially. Very few dwarf species are offered, except in the seed exchange of the AGS and other kindred societies. You can, of course, collect seed from your own plants if you already have a collection. But, be aware that if you grow a range of species in close proximity there is always the possibility that hybridisation has occurred. This may well also have happened when getting seed from elsewhere. Not knowing quite what you are going to get may or may not be of concern to you. After all, some of the best hybrids have turned up in gardens in this way. My own hybrid, R. ‘Bryn Mair’, illustrated here, is a good example.
If you are growing plants from your own seed you will need to wait until it is ripe (as indicated by the splitting of the capsules). The exact time varies with species but is rarely before the end of October in the UK. I generally store the almost dust fine seed in small paper envelopes in a cool place until sowing in February-March. Rhododendron seed is generally short-lived and germination rates decline rapidly, so it is unwise to delay sowing. This should be borne in mind when purchasing rhododendron seed, or obtaining it from seed exchanges.
Having forsaken peat use in the garden for conservation reasons, I now use equal parts (by volume) of finely chopped live sphagnum moss and Perlite. The moss is available at most garden centres, but I collect mine from the hillside behind our house. Fill the pots (I use 10 cm square half-pots) and firm gently, then drench with boiling water to sterilize the compost. When it has cooled, sow the tiny seed very thinly on the surface. This is to avoid overcrowding of the seedlings that will inevitably lead to fungal infections.
As the sterilizing process thoroughly wets the sphagnum moss there is no need to water again at this stage. Place the pots in a tray with a plastic dome cover to maintain high air humidity. A polythene bag with wire hoops to keep it away from the seed works equally well. Put the trays or pots in a shady spot (e.g. beneath the glasshouse staging, or in a well shaded frame with at least 50% light reduction). It is not usually necessary to water again until germination occurs, generally one to two months later. However, if the sphagnum shows signs of drying out, spray generously with tepid water, then replace the cover.
If you are able to provide a little gentle heat to the trays or pots, for example a heated mat with thermostatic control (20C), this will speed up germination and subsequent growth, but is not necessary.
When still at the first true leaf stage, to avoid undue root disturbance, I prick out small batches of seedlings into pots or seed trays. I put them in the same sphagnum/Perlite mix, returning the pots or trays to the shaded area after watering with a quarter strength solution of ericaceous soluble fertilizer.
Applying a fungicide at the strength recommended for prevention of ‘damping off’ diseases is worth considering at this stage.
I separate the batches of seedlings into individuals as soon as they are easy enough to handle. At this stage the seedlings need to grow on rapidly without a check, so I put them in an ericaceous compost containing low levels of added nutrients. Gradually reduce shading as they start to grow away. Provided they are grown on without a check, seedlings will sometimes flower in 2 years, often in 3 or 4.
As a generalisation, cuttings of dwarf rhododendrons root more readily than those of their larger kin. Also they flower sooner, generally in 2-3 years. Unless you have a mist bench, or other specialised facilities, in my experience semi-hardwood cuttings are the easiest and quickest to root. I take these soon after the new growth has stopped expanding and while it is still pliable, but not soft. This usually means mid-July to early August here in Wales. But like much else concerning propagation, experience is the best guide, and you get to know when the new shoots are ready to take.
I do not find that a heel of older wood makes any difference to success rates. Therefore, I use the new ‘wood’ only. It is essential to use a sharp knife. Remove any flower buds, dead leaves and lower leaves that would otherwise be buried in the rooting compost. It helps rooting to ‘wound’ the base of the cutting. I do this by removing a small sliver of bark from opposite sides of the cutting.
If you wish to use rooting hormones, now is the time to apply them. For rooting powder, first dip the bottom 1 cm of the cuttings in water. Shake off any excess, and then dip them in the powder. For hormone rooting gel, hold the cuttings with their bottom ends in the material for at least 30 seconds, preferably a good deal longer.
Insert the prepared cuttings closely together in rows in the rooting compost. I normally use equal parts of fine composted forest bark, sieved (sterilized) leafmould, and Perlite. Water thoroughly, cover the trays or pots, and place them in a shaded cold frame. Only water again when the need is apparent, which comes down to experience. As a guide, though, it is definitely better for the compost to be slightly dry rather than too wet.
Cuttings taken in July-August should be sufficiently well rooted to pot up before the onset of winter. Tardy rooters can be left until they show signs of renewed growth in early spring. Expect huge differences in rooting percentages between different species, hybrids and cultivars, that is par for the course. I generally consider anything over 20% acceptable and over 50% good.
If you cover the shoots of dwarf rhododendrons with soil, they often form roots. This is known as ‘layering’, and the resultant plants as ‘layers’. These are an easy means of small-scale increase if you only want a few plants. If you top dress rhododendrons in the spring after flowering, they may produce rooted layers by the following autumn, or it may take longer.
If a good mass of roots has been produced, I cut through the stem between the parent and layer and then leave alone for at least six months. This allows the layer to become independent of the parent without undue root disturbance. Then It is an easy matter to carefully dig out the layer and either pot it up, or replant in fresh soil. In the latter case, it is best to do this in the autumn, to avoid the risk of drought and heat damage. In either case, it is important to keep the layer well watered, and in hotter climes some shading may be advisable.
Rhododendrons may be increased by grafting, which is the production of a union between a shoot of the chosen variety (scion) and a rooted seedling (stock). This was once common practice for difficult to root larger rhododendron cultivars. But with the the advent of improved techniques and equipment for rooting cuttings it has become much less common.
Grafting was never much used for dwarf rhododendrons, in part because, as explained earlier, cuttings are easier to root than those of larger varieties. While I have tried my hand at grafting with a number of other dwarf shrubs, notably daphnes, I have never felt the need to use it for dwarf rhododendrons. Neither, I suspect, will you. But there are plenty of books and articles on grafting in general if you wish to give it a try, so I shall say no more about it here.
Rhododendron imperator (syn. R. patulum)