Growing alpine plants in the age of climate change

How to be cool

In general, mountains are cold places. The ambient temperature drops by 5° C for every 1000m altitude. Alpine plants are thus adapted to grow slowly in cool conditions, and many have metabolic systems that are unable to withstand high temperatures. This is particularly true of plants from the Himalayas and western China. Here they experience monsoon clouds and rain during their summer growth periods, so that they rarely overheat in their native habitats. However, in our gardens alpines are increasingly exposed to periods of excessive summer heat as the climate warms. I know that some of my friends who garden in the south of England no longer attempt to grow many ‘classic’ Himalayan alpines, primulas, meconopsis and autumn gentians for instance because they fail during hot periods. They find that it is not enough to keep the plants well-watered. Many alpines die from heat-stroke.

I am fortunate to live in north-east England. Up to now we have yet to suffer most periods of extreme summer heat (over 28° C) experienced by the rest of England and Wales over the last few decades. Indeed, there have been few notable changes to our climate during the half-century I have lived here. I am happy to report that we can still grow many Himalayan and Chinese alpines with some moderate degree of success. Nonetheless, I would not like to give the impression that any such achievement is readily attained. Rather, I would like to think that as much thought, cunning and planning is involved as for those who succeed with dionysias (which seem quite impossible in my conditions). This short article describes some of these strategems to help grow alpine plants despite climate change.

Some simple physics

Three principles control the temperatures which our plants experience:

• Radiation (from the sun). Heat is acquired from radiation (e.g. an electric fire), convection (e.g. a fan heater) and conduction (e.g. an electric blanket). The heat convection and conduction experienced by a plant will depend on the ambient (shade) temperature, but we can protect our plants from radiant heat by simply placing them in the shade.
• Air density. Cold air is denser than warm air, and sinks. The coldest part of the garden is usually the lowest, particularly if it is sheltered and the air is trapped.
• Latent heat of evaporation. When water evaporates, it takes energy, thus taking heat from the surface it evaporates from. This why we sweat! If you spray plants, it will cool them; likewise, the more they transpire, the cooler they get.

Thus, during a hot spell, because we live on a gentle north slope, I make sure that vulnerable subjects are moved to shaded sites near the bottom of the garden. They are sprayed here with a hose, preferably in the evening. Any plants which normally live in the alpine house and are susceptible to heat are moved outside to a shady spot (if rain-tolerant), or placed on the floor of the alpine house.

Vulnerable subjects

So, what do I consider to be vulnerable subjects, and how are they moved?

Amongst the plants I consider to be particularly susceptible to heat-stroke are

• many Himalayan, Chinese and Caucasian primulas (but not species in sections Proliferae, Sikkimenses, Denticulatae or Oreophlomis)
• some small meconopsis such as M. delavayi, M. balangensis, M. integrifolia, etc.
• all Himalayan and Chinese gentians
• omphalogrammas
• most saxifrages in section Porophyllum
• some Himalayan and Chinese androsaces
• Chilean forest-dwellers (Philesia, Sarmentia, Asteranthera etc)
• Himalayan Gesneriads (petrocosmeas, etc

There are many others, and any grower will be able to fill in some blanks.

Mobility

When trying to move plants around the garden, you can make your life easier by using the following strategies.

• Grow susceptible subjects in pots. This has its drawbacks because plants need to be repotted regularly and often benefit from liquid feeds. However if they can be removed to a cool and shady spot during a heat crisis it will often save their lives. I grow almost all such subjects in plastic pots. These are lighter, easier to judge the need for watering, dry out less often, and are less likely to tip over.

• Create mini-gardens in expanded polystyrene containers. I grow many plants in repurposed fishboxes, packing materials, laboratory flask containers and so on. These are light enough to be moved quite readily, although this is less true of the larger versions, fully saturated and with plentiful stony top-dressing. I used to move these containers about quite regularly, taking them into full shade during hot spells until Anno Domini and a bad back intervened. Now those containing heat-susceptible subjects reside in dappled shade under deciduous trees, where they will receive more light during our gloomy short winter days.
• Another strategy is to grow plants immediately under a low north wall. In the photo below, young plants of Omphalogramma forrestii grow best immediately under the wall. The plant furthest out is showing the most heat stress.

Humidity

It is inevitable that cool dark places that are regularly sprayed will be humid. High levels of humidity do not necessarily reduce ambient temperatures however. Plants transpire less into humid air, and so benefit less from the latent heat of evaporation.

As long as they are watered regularly, the more plants transpire, the cooler they get. A damp corner will be cooler than a dry one because of evaporation, even if the plant remains less so.

Renewing a polystyrene ‘trough’

As intimated above, I have about 40 containers, ‘troughs’ if you like, made from expanded polystyrene. I find that if the contents are to remain pleasing to the eye and the plants in good health, the planting needs to be refreshed every 5-7 years. Now that it is cooler and raining regularly, and plants are still in growth, it is a good opportunity to get this done.

The ‘trough’ in question is one of the smallest and lightest I have and was planted about six years ago. The base was originally some packing material in a parcel and is only about 12cm deep outside and 9cm deep inside. As with all my polystyrene containers, it was painted with stone-coloured plasticated paint. Then dry coarse sand was thrown at the paint while still wet, for a more natural look.

Because the trough was so shallow, it was built up with a piece of home-made hypertufa. I used my own recipe using shredded lawn moss, perlite and cement. The piece was left to mature outside for several months. (For further details on how to make your own hypertufa troughs and rocks visit our dedicated page.)

This was sawn in half and the halves ‘planted’ upright so that a vertical crevice became available between the two halves. The hypertufa rock allowed compost (John Innes 3, grit and perlite) to slope up to a depth of 15cm or more and, remarkably, few if any subjects had been lost as a result of drought. Most had rooted into the hypertufa which absorbs and retains moisture very efficiently. However, the trough had become overrun with a variety of mosses and liverworts (testament to its moisture-retention) and most of the plants were no longer thriving.

A list of survivors follows:

• Androsace elegans (no longer flowering)
• Campanula chorhuensis (never flowered, eaten by slugs)
• Daphne x hendersonii ‘Marion White’ (grafted onto a D. mezereum seedling using a scion from Alan Furness)
• Dianthus simulans (just surviving)
• Dianthus ‘freynii’ (possibly this but non-flowering)
• Erinus alpinus (which has seeded profusely onto the vertical hypertufa piece)
• Gentiana ligustica
• Saxifraga ferdinandi-coburgii

The first task was to carefully strip away all the moss and liverwort, while attempting to retain all the ‘tufa’ (some of which is quite soft). Both dianthus, the campanula and the original large erinus were removed, so that only four well-established plants remained.

The gaps were built up with new compost, and a variety of young plants were introduced:

• Aciphylla crosby-smithii (from seed)
• Daphne domini (seed)
• Dianthus haematocalyx pindica (seed)
• Draba bryoides (cutting)
• Dianthus pavonius (seed)
• Dianthus tymphresteus (seed)
• Pulsatilla vernalis (self-sown seedling)
• Saxifraga ‘Lozjicka’ (cutting)
• Saxifraga sempervivum (cutting)

To finish, the whole was carefully top-dressed with fine grit, and watered with a fine rose, probably unnecessarily as it seems to have rained ever since!