Heat stress and roses
by Carolyn Elgar
Whether or not you believe in global warming, you can certainly agree that the past month
has been a hot one. Consistent temperatures in the high nineties is enough to make any
gardener escape to the beach, the pool, or the coolness of the air conditioned living room.
Rose care actitivities fall lower on the priority list when we have to sweat and battle bugs in
these humid, hot temperatures.
Unfortunately, our roses
don’t like lots of intense heat either.
Ask any rose gardeners who live
in the desert and they will tell you
that their bloom production falls
off dramatically after the month of
May. Roses are not tropical plants.
Sun damage shows up on roses as crispy and
blackened leaf tissue. Proximity to hard surfaces such
as concrete walls, sidewalks, and siding intensifies
the heat.
The foliage can actually burn, especially if the plant
is experiencing water stress as well. Bloom size and
production are strongly affected by heat. As the plant
struggles to stay hydrated, fragrance diminishes and
blooms are small.
Dark blotches and crispy edges on the leaves are a sign of a heat
stressed plant - leaf tissues suffer as the plant conserves water.
Water needs increase dramatically, to the point where
daily watering is necessary. Potted roses may need
even more water as the soil heats up rapidly when it
is confined to a container. It is essential to check soil
dampness frequently during hot days. Use a water
probe or poke your finger into the soil to a depth of
around six inches.
High temperatures affect the biological processes of
photosynthesis. When the roots can’t take up enough
water, the leaf tissue furthest from the veins will
dehydrate and crisp. There is plenty of debate as to
whether you should remove the desicated leaves;
many rosarians claim that these leaves shade the
canes, protecting them from dehydration. Dead
leaves are replaced easily with new ones when
the temperatures improve, but once a cane dies,
it takes time for a new one to grow.
Orange County Rose Society
September 2012
Small leaves dry out quickly in hot sun.
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Another effect of heat on photosynthesis is the depletion of chlorophyl, which is the stuff that makes
leaves green. As a result, yellowing occurs. Again, the plant will conserve its resources for roots, canes,
and leaves, in that order.
Spider mites are the major destructive insect pest you will encounter on hot days. These hard-to-see
insects live under the leaves and rasp the tissue. Heat increases their reproduction; in no time they
can completely defoliate a bush, especially a miniature rose that is close to the ground. Look for dirty,
stippled looking leaves and, in severe cases, webbing on the leaves. Don’t spend a lot of time actually
looking for the tiny insects. The damage you can see is enough to let you know they are there.
You can help your struggling plants in a number of ways. Move potted plants into a cooler area, such as
under a patio cover or a shady tree. Some desert rosarians use light colored umbrellas, reflective foil, or
shade cloth. You may want to shade an entire area of the garden, hot spots such as walls, or individual
plants. The important thing is to make sure there is plenty of air circulation. Avoid dark shade cloth as it
will actually raise the temperature around the plant.
Spray the plants with water in the morning when
temperatures are hot for long periods. Jets of water
can blast dirt and dust off the leaves and hydrate
them in preparation for a hot day. If you use a strong
water wand, you can dislodge spider mites. Because
they reproduce so quickly, you will need to do this
every few days. Aim the water under the leaves
where they grow and lay their eggs.
Above all, do not fertilize during
hot periods. Even organic nitrogen
can burn the roots of a stressed
plant. Like people, when it’s hot,
roses want lots of water and heat
relief rather than food.
Group plants in a shaded area for improved humidity for the leaves.
And check your watering system every day. One
broken sprinkler head or chewed up drip tube can
result in a dried out, struggling plant.
The Rose Gazette is in the process of redesign
as your editor attempts to implement a new,
fresh, more casual look. More changes to come!
Orange County Rose Society
September 2012
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