Wilting in plants has
many causes
Brian Jervis: Ask a Master Gardener
Saturday, July 9, 2016
Q: What would cause a shrub in my yard to
wilt? I am watering it a lot, and it is still wilted. Carl, Tulsa.
A:
Most gardeners associate
wilted leaves with too little water and often that is the case. However, there
are other causes of wilting for plants in the landscape and in houseplants.
To understand wilting, one must understand a
plant’s circulation. Absorption of water in soil depends on having an adequate
number of tiny hair-like roots. These absorb water, as well as nutrients and
oxygen. After absorption, water is transported to the leaves by the cambium, a
layer of tissue just under the bark. In the leaves, the pressure of water
supports the leaf structure. Any interruption of the flow of water to the
leaves results in loss of structural support, resulting in wilting.
It is not often recognized that too much water
can produce the same effect as seen in a drought. Excessive water forces oxygen
from the soil, which results in suffocation of roots which, oddly enough,
prevents water absorption. Initially, this results in wilting, followed by
yellowing of leaves and, if prolonged, death of the plant.
Protracted wet soil may be associated not only
with changes described above, but also a higher incidence of root fungal
diseases, diseases which love wet soil. These diseases, collectively called
“root rot,” can kill the plant or lead to poor performance in the future.
Other conditions interrupting a plant’s
circulation may produce wilting. These include temperature-related problems,
either too hot or too cold. Hot weather causes water loss to exceed the ability
of the plant to keep up with demand. Hot and cold weather may also damage roots
in some plants. Mulch will help prevent this.
Excessive salt in the root zone, such as is seen
from overdoing fertilization or from de-icers in winter, it will damage roots
and cause wilt. Salt accumulation from fertilizers in houseplants is also a
common cause of wilting. Periodic flushing of the potting soil with water will
prevent this.
In addition, any condition that interrupts the
flow of water in the above ground portion of plants will result in wilting. This
includes circling roots, which strangle the plants, grass trimmer damage and
string and wire bindings used to stake a tree or hold a root ball together.
One type of wilting not mentioned and that is
reversible and insignificant is the wilting occurring in evergreen plants when
it is cold. This is a way some plants have of protecting themselves from the
cold, and it reverses with warming.
All of the above may cause wilting with or
without progressive damage to plants. It is important then to try to determine
what the problem is before launching into a vigorous watering program.
Master Gardeners can help you sort out the
problem. Call their office helpline at 918-746-3701 or bring samples of plants
into the office at the OSU Extension Center at gate #6 at the Tulsa
Fairgrounds, 4116 E. 15th St.
Webworm alert
We are beginning to see large numbers of fall
webworm nests in pecan, persimmon and occasionally redbud and other trees.
These pests, as the name suggests, are usually more prevalent in the later part
of summer and into the fall. This year’s early outbreak has prompted Eric
Rebek, entomologist at OSU, to issue a “Pest e-alert” document, accessible
online.
We had the same problem last year, and Rebek
feels the large numbers of webs both years is related to a mild winter and wet
spring.
These caterpillars build dense web nests around
leaves on the ends of tree limbs. They devour the leaves within the webs and
after leaves are eaten, expand locally to include new leaves. Even with large
numbers of webs in pecans, the pests usually do not damage mature trees (in
pecan orchards nut production may suffer). Although they are unattractive,
mature trees will survive, but heavy infestations may injure or kill smaller
recently planted trees.
Control is difficult. There is no effective way
to predict or prevent the pests. Even though there are large numbers of
parasites and predators, they are unable to control large outbreaks.
On smaller trees, the webs may be pruned out or
removed by hand. Insecticides are usually not warranted. If a synthetic general
purpose insecticide is used, it needs to be sprayed with enough force to enter
the web. However, this is not practical or safe, especially in an urban
setting. An alternative is to use one of two relatively safe organic insecticides —
Bacillus thuringiensis (Bt) or spinosad. They are sold under several different
brand names. These are sprayed on leaves next to the webs and will be consumed
as the webs expand
Contact the OSU Tulsa Master Gardeners for more
recommendations. Call 918-746-3701 from 9 a.m. to 4 p.m. Monday-Friday.
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