The Grandest Rhododendrons

Although the first rhododendrons started to bloom in July, October is really their month, and with over 800 species and countless cultivars there are so many that it can be hard to pick a favourite plant or style. But for me one group of October-flowering rhododendrons always stands out — the grandes.The Grandia subsection of the genus Rhododendron includes some 12 species and two varieties, all commonly known as grandes after Rhododendron grande, the type species. They are: R. balangense, R. grande, R. kesangiae (including varieties kesangiae and album), R. macabeanum, R. magnificum, R. montroseanum, R. praestans, R. protistum, R. pudorosum, R. sidereum, R. sinogrande, R. watsonii. The plant formerly known as R. giganteum, one of which may be seen at Isel Park, Stoke near Nelson, is now included in Rhododendron protistum according to recent revisions.They are all marvellous tree-sized plants with large flower trusses in colours ranging from white through yellow to purple-pink, usually in pastel shades. Most often however, their real appeal lies not so much in their flowers as their foliage. They have huge, leathery, elliptical leaves that are deep green above with pale buff to rich tan indumentum below. The leaves are the largest found among the rhododendrons and have quite a tropical appearance, but despite that the grandes can all withstand at least –6°C and most will tolerate –10°C once established.Their natural distribution covers the classic Himalayan rhododendron zone from western Szechwan and Yunnan through northern Myanmar (Burma) to Assam, Manipur and south-eastern Tibet at 2,500–4,500 m altitude. That’s pretty high, but in the Himalayas it’s only mid-range. It’s an area that because of its near constant cloud cover is known as the cloud forest. The clouds not only provide ample moisture they also significantly lower the light levels, so a large leaf area is required to make the best use of the limited light available for photosynthesis. That need combined with no reason to evolve mechanisms to cope with drought has led to some very large-leafed plants indeed.The grandes generally prefer a cool climate, and as might be expected of plants that naturally occur in some of the world’s rainiest places, they like regular moisture. If you can provide a deep, humus-rich soil with plenty of moisture and some shade while the plants are young, they’re not difficult to grow. Although one species, R. protistum, is a little tender, they thrive in mild to cool, moist conditions but suffer in low humidity or if hit by repeated hard frosts.Except as seed, few grandes are readily available and just two species, R. macabeanum and R. sinogrande, are at all common. They both have cream to pale yellow, bell-shaped flowers in rounded trusses of 15–30 blooms and very large leaves. Those of R. macabeanum reach 45 cm long, while a mature R. sinogrande can have leaves up to 85 cm long, the largest of any rhododendron. Rhododendron macabeanum is very adaptable and particularly easily cultivated.Keep an eye out for the grande rhododendrons, I’m sure you’ll be impressed.Preparing your rhododendrons for summerAs the weather warms in early spring it's time to make sure you have plenty of good compost or mulch at hand, preferably a type rich in humusy leaf mould. Because before you know it’ll be time to think about what happens to your rhododendrons after they’ve flowered. It’s important that you don’t just forget about them until next spring because the quantity and quality of next year’s bloom depends on what happens over the coming summer.Evergreen rhododendrons usually have two spurts of growth, one in spring and the other in autumn, and the flower buds begin to develop from late January. Any growth checks due to drought or general poor plant condition are bound to influence next spring’s flowering.The first step is to remove as many of the old flower heads as you can reach. There’s no point in wasting the plants’ valuable reserves on seed production — unless you want to sow a few seeds. Removing the flower heads as soon as they have faded also prevents fungal diseases developing among the decaying petals.Because rhododendrons are shallow rooted with very fine roots that are easily damaged by changes in soil moisture and temperature, they benefit more than most plants from regular mulching. The mulch conserves moisture and prevents the topsoil getting too hot or being blown away in summer, and it also reduces the effects of winter frosts and driving rain. A loose layer of mulch makes weeding easy, which means that you don’t have to risk root-damaging cultivation around your rhododendrons.It’s best to use fairly coarse mulches, such as leaf mould, well-rotted compost, bark chips or shredded bark, that allow easy passage for air and moisture. The mulch should be around 5 cm deep in summer and about twice as deep in winter if you live in an area where heavy frosts or soil scouring rains are likely. In spring use a rake to redistribute the winter mulch back to around a 5 cm deep layer. You’ll find that the mulch naturally decomposes and needs to be topped up occasionally, but don’t mound it up around the main stem as this can cause the bark to rot.As the temperatures rise in summer make sure there is plenty of moisture beneath the mulch. Because the mulch looks much the same at any time it’s very easy to forget that the soil under it can dry out. Regular watering in dry weather is essential, but remember to water deeply so that the roots don’t become confined to just the surface portion of moist soil. As always, occasional deep watering is better that frequent shallow doses.As soon as spring arrives and the flowers begin to open you can apply fertiliser. If your soil is inclined to be alkaline use sulphate-based fertilisers as these will help to keep the soil pH at around 5.0 to 6.0, which is what most rhododendrons prefer. Garden centres usually stock specific rhododendron and azalea foods or acid plant fertilisers, though any fertiliser with an analysis of around 6-10-4 or 5-10-5 should be fine.Be sparing with powdered fertilisers because rhododendron roots are easily burnt. Always water the fertiliser in thoroughly to lessen any risk of burning. Also, don’t apply nitrogen-rich fertilisers after New Year or you may find in the autumn that the plants tend to produce masses of foliage at the expense of flower buds.While mulching and applying fertiliser it’s a good time to see if the plants need any pruning, thinning or shaping. Except when vastly overgrown plants need rejuvenating, rhododendrons rarely require anything more than thinning and trimming to shape.Do any pruning immediately after flowering so the plants have the maximum time to put on new growth before the next bud-forming season. That way you won’t lose too many flower buds. Start by removing any overly long stems and water-shoots, then thin the centre of the bush slightly to allow good light and air penetration. Finally, pinch back the growing tips of any vigorous young shoots to encourage bushiness.Very overgrown rhododendrons can be rejuvenated over two or three seasons by cutting the long branches back by a third to a half each spring. Try to avoid cutting the branches back to bare wood as this is often reluctant to reshoot.The chances are that the plants won’t flower very much during this treatment but at least it won’t have to replaced. After three or four years the old shrubs will once again be dense, compact and bushy and you’ll need to think about thinning some of the mass of basal growth that has developed.As summer turns to autumn you can cut down on the watering and allow the plants to dry and harden for the coming winter. Although rhododendrons will often put on large amounts of autumn growth, be sparing with fertiliser if you apply any at all. Otherwise you may find that winter frosts are more damaging that they may otherwise be. Not perhaps to the extent of killing the plants but perhaps destroying some of the flower buds and that late young growth.A little attention to summer care will be rewarded in the spring.

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