Plants Feel the Heat Too - Unitec's Sue Wake gives advice on helping beat the weather wilt
Plants Feel the Heat Too
As the mercury hits a high throughout many parts of the country, it’s not just humans and animals who are suffering in
the heat. Sue Wake, senior lecturer in Landscape and Garden Design at Auckland’s Unitec, gives some tips on how to help
your plants stand up to the extremes in weather:
Things you might not know about plants and (hot) weather:
Plants transpire (equivalent to our sweating) through tiny pores in the leaves called stomata. This cools them and also
draws water up the stem – circulating it around the plant. Unfortunately, about 90% of water they take up is lost to the
plant this way – so they are at risk of ‘water stress’ if they can’t replace the lost water through uptake from the root
system. Even well-established trees will suffer if the hot weather continues too long, so don’t forget to water trees on
your property during prolonged dry spells (eg more than 2 weeks of no rain in Auckland in hot summer temperatures – more
often for trees less than 3 years old). Protect your shade investment!
Do plants get sunburnt?
Yes, they do – if the plant is not acclimatised to bright light/sun, or if it is not adapted to it, it will suffer sun
scald on leaves, which is ugly and reduces the productivity of the leaf (ie, less photosynthesis). So don’t put indoor
plants that are adapted to low light (eg, understorey tropical plants) into the sun, and equally, don’t put your indoor
basil plant that is used to filtered indoor light – straight out into the sun.
Plants that are adapted to hot and dry conditions (eg deserts) have special adaptations such as reduced leaf area (think
cactus spines) and they may even have evolved special devices to protect them from the sun, eg red pigments and silver
colours. Often leaves that look silver or grey are actually covered in fine hairs that both reflect the light (like a
sunshield) and provide a barrier to water loss out through the pores. These plants are often good for coastal areas –
coping with both sun and salt-laden winds.
When’s the best time of day to water?
Water when the plant is best able to take up the water, without further losses. For example, early morning or early
evening. When it is this hot and plants are really struggling, especially in pots and planters, water them morning and
evening. Watering in the middle of the day when plants are out in the sun, is not recommended since water sitting on
leaves can cause sunburn and the plant can’t make such efficient use of water received then. For water economy and
efficiency, set irrigation timers for 2am – pressure will be highest since most people are sleeping, not showering, and
it is cool enough for the water to have greatest effect on the plant. At that time of the day the plant is not carrying
out photosynthesis so the pores in the leaves are closed and the plant can ‘top up’ its water content.
Water sitting on leaves can lead to fungal infections, for example, powdery mildew. This is worst in warm weather and
humid weather. Air flow is important to reduce it – with edible plants, make sure plants are spaced far enough apart to
allow air flow gaps and remove leaves and stems that are causing ‘crowding’. Sprays made of baking soda and detergent
can also help if you have problems with powdery mildew (grey powder over leaves and flowers).
Should I also be feeding my plants in the heat?
Plants will burn more energy to survive and grow in hotter weather. As long as they have sufficient moisture they will
generally grow faster in hot weather and they often flower earlier. However, they also need more nutrients, especially
as fertilisers will be more readily leached (lost) from the soil when plants are being watered more often. Using slow
release fertilizers is recommended when growing in pots, these release nutrients over time, although they will release
more quickly when the soil is warm. Watch for any signs of general nutrient deficiency such as yellowing of lower
leaves. For pots and planters, using liquid feed fertilisers is a good supplement to the slow release fertilisers
already in the potting mix. Mix these with water as per directions and water the mix with this. Your hot and thirsty
plant will thank you for it!
Lookout – here comes the wind again!
Apart from increased disease risk, most plants actually like humid conditions since this means more moisture in the air,
which reduces their own water loss (lowered rates of transpiration). It is wind that is most detrimental to plants –
stripping away more water by transpiration and damaging leaves and branches.
Unitec offers a range of full and part-time programmes and short courses in Landscape and Garden Design. For more
information, see Landscape and Garden Design courses