Plant resilience

Sunday, 02 February 2020

Agapanthus praecox subspecies orientalis in my driveway

In the midst of this truly awful summer, there are very few things that provide much cheer; however, the resilience of some of our plants in the face of the extreme heat and drought is quite astounding. It's certainly the case that I have lost quite a few plants this summer, but many others are proving to be tough customers. I plan to replant much of my garden this coming winter in order to utilise more of these stalwart specimens.

Justicia brandegeeana Lutea, from Mexico, thrives in Sydney

Whilst determining which plants are going to be keepers in this era of longer, hotter, drier summers is going to be a matter of trial and error, along with close observation, I am interested in looking for patterns and themes in the most stoic of the survivors. One obvious approach is to look at the provenance of plants. My main casualties this summer have been the remaining cool-climate specimens that I still loved to grow from my cottage garden days, when I thought any plant could grow anywhere, if I wanted it to. Bitter experience has taught me that, funnily enough, plants that hail from snow-capped mountains in Europe or the Himalayas don't do that well in hot old Sydney. My garden now chiefly contains plants that come from warm temperate or subtropical regions of the world! Those from South Africa, South America (but generally not the mountainous areas), Mexico, some parts of Asia, southern China - and, of course, plants native to the Sydney region - on the whole do very well. Mediterranean plants are a bit hit and miss, as many dislike our summer humidity, but as our summers are getting hotter and drier, they are likely to do better than when we had a lot of summer rain.

Artemisia Powis Castle (left) with Plectranthus argentatus

There are a number of plant characteristics that are associated with heat and drought survival. Leaf types are often a good indicator. Silver or grey foliage reflects light from leaves, resulting in less water loss from transpiration. In many cases, these leaves have an interesting texture from other features to reduce transpiration: such as a protective layer of down or hairs to trap moisture and reduce the effects of wind (giving a velvety look) or a surface wax (which gives a polished finish). Silvery-leaved plants that have done well for me include Artemisia 'Powis Castle', Salvia discolour, Lychnis coronaria, Helychrysum petiolare and Plectranthus argenatatus (the last two plants do well in shaded areas).

Salvia Marine Blue has survived well in my garden

Having small leaves is another plant adaptation to drought and heat, as this gives less surface area on foliage and hence reduces water loss from transpiration. I have definitely noticed that my small-leaved Salvia specimens, such as Salvia microphylla cultivars and hybrids, Salvia muirii, Salvia 'Marine Blue' and Salvia semiatrata are surviving very well. The larger-leaved Salvia are going OK, but are wilting more on hot days. Other plants with compact foliage include many types of daisies (Erigeron, Argyranthemum (Marguerite daisies), Aster and Tagetes), Linaria purpurea, Gaura, Verbena and grassy-leaved plants in general (such as Miscanthus, Poa and Lomandra).

Rose form of scent-leaf Pelargonium

Aromatic leaves are said to contain scented volatile compounds that appear to increase the air density around them and cool foliage as they evaporate. I certainly have noticed that a number of my summer survivors have aromatic leaves: rosemary, Nepeta, Salvia fruticosa, scent-leaf Pelargonium and oregano, though they often also have some of other leaf features already mentioned as helping plants to cope with drought and intense heat.

Crassula multicava

Succulent plants are an obvious choice for drought and heat tolerance, as their stems, leaves and sometimes their roots have the ability to store moisture to help survival. Some of my favourite genera include Kalanchoe, Crassula, Schlumbergera (zygocactus) and Sedum. These need to be planted in a spot with good drainage (or in pots), as heavy rains that occur in storms can lead them to rot if the water cannot get away. Semi-succulent plants include the wonderful array of Begonia, ranging from groundcovers to hefty shrubs, and the various forms of Pelargonium. All have endured this summer very well!

Zephyranthes minuta has recently flowered after the rain

Particular adaptations of roots can also signify plants that cope well with heat and lack of rain. Thick, fleshy roots or rhizomes are a feature of some of the good doers in my garden over summer: Agapanthus (though their leaves were scorched on the 47-degree day in January!), Clivia, Alpinia, and Chlorophytum. Some fleshy-rooted plants become dormant in the heat: Acanthus and Alstroemeria hybrids - these will reappear later in the year. Of course, bulbous plants are also well adapted to drought, being dormant during summer. The pretty rain lilies (Zephyranthes species and cultivars) flower whenever rain does fall over summer despite being leafless most of this season due to lack of water! The gorgeous belladonna lilies (Amaryllis belladonna) from South Africa are just starting to send up their trumpet flowers now, unscathed by the summer heat. South African spring bulbs survive very well in our climate, and include Freesia, Babiana, Ixia and Sparaxis.

One mechanism that plants can use to cope with heat and drought is to slow their growth: I have noticed this happening with many of my plants. After the rain two weeks ago, they perked up very quickly and began growing again. The heatwave this weekend has slowed them down again; I live in hope of further rain this coming week! In the meantime, I salute those plants in my garden that have managed to live through these dreadful times, and plan to spread them through my garden.