As everyone now acknowledges, climate change is affecting the way we garden. Rhododendrons and azaleas are shallow rooted plants and therefore periods of hot weather and drought affect them, especially when recenty planted or where there is competition from tree roots.
Camellia and Magnolia once established are deep rooted and can usually find moisture where the more shallow rooted rhododendrons can struggle.
Many rhododendrons, particularly those from the Sino-Himalayan region are used to summer monsoon conditions and never dry out in the growing season in the wild. Rhododendrons from Japan, N China, Vietnam, N America and the Caucasus are more adapted to hot and dry weather. Most azaleas in particular show considerable sun and heat tolerance.
High mountain sino-himalayan species such as those in Section Pogonantha and Subsections Campylocarpa and Taliensia have roots which are very susceptible to root pathogens in hot summer soil, particularly soils which have to irrigated. These species prefer a cood root run and thrive best on the north side of high buildings or trees to protect from sun in summertime. Shade from trees also helps but not if the trees are greedy ones: beech, syamore, limes, maples, leylandii etc take all the moisture out of the soil. Sorbus, oaks, larch are deeper rooted and dont cast such dense shade, so are better bedfellows for rhododendrons.
Hosepipe bans are likley to become more common in S.E. England and perhaps elsewhere. Cornwall, Wales, W Scotland and Irish gardens have usually worked on the assumption that enough rain will fall from the sky, so irrigation is seldom available. Recent summers have forced us to think again.
1. Select varieties with a degree of heat and sun tolerance: most hardy hybrid rhododendrons, most azaleas, dwarf rhododendron hybrids (but not species). Yellow flowered larger rhododendrons have less heat tolerance than other colours.
2. Triflora rhododendrons, Maddenia and azaleas are more tolerant of drought, once established, then species rhododendrons.
3. New plantings in particular will need to be irrigated in the first season or two. Mulching can help conserve water in the soil.
4. As well as sprinkers, consider using seep hose with tiny leaking along the whole length... or micro irrigation with small sprinklers which produce fine spray which leads to less wasteful run off.
5. Water can be recycled, if it runs downhill into a pond for example. But be aware that you can end up moving fungal disease around the garden if you are not careful.
6. Avoid planting under or near greedy trees like beech and sycamore. They will always win the battle for light and moisture. Camellias and magnolias are greedy and will tend to punish neighbouring shallow-rooted rhododendrons.
7. Grafted plants on rootstocks with heat and drought tolerance often produces more resiliant plants than those on their own roots.
8. If plants are badly burned don't panic. Water and liquid feed them... to encourage some new growth... and they will often recover fine.
