Thinking Ahead:
How to Optimize Next Season’s Plant Health this Fall and Winter
Fall and winter are the times of year when our thoughts of outdoor gardening
activities often become a distant memory. However even during the late fall and
winter, there are basic outdoor gardening tasks that need to be completed in
order to prevent potential disease problems in the coming spring and summer.
Plant Debris Cleanup
Leaf diseases such as anthracnose (see UW Garden Facts
XHT1001a/b/c
– see below for link to Garden Facts website), Tubakia leaf spot (see UW Garden Facts
XHT1104), apple scab (see UW Garden Facts
XHT1007a/b), and tar spot (see UW Garden Facts
XHT1126) were very prevalent on woody ornamentals in 2004, in part due to
the extremely wet weather early in the growing season. Most fungal leaf
pathogens overwinter in leaf litter, and thus leaf litter can serve as an
important source of fungal spores during the following growing season. In
addition, herbaceous plant debris is an important overwintering site for
herbaceous plant pathogens. Therefore, be sure to rake up tree and shrub leaves
after they have fallen from your woody ornamentals, and be sure to cut back and
remove herbaceous plant debris after the vegetation has been killed by frost.
There are many options for disposing of leaves and other plant debris. Many
cities offer drop off sites for organic waste and this waste is eventually
recycled by composting and then provided back to the community either free of
charge or for a small fee. Home composting is also an option for disposing of
plant debris, even when that debris contains plant pathogens. The combination
of heat and degradation of plant tissue provides an environment where most plant
pathogens will not survive. Burying can also be a useful technique for
disposing of leaf and herbaceous plant debris as long as there is a layer of
soil separating the debris from the outside environment. Burying promotes decay
of plant tissue, which, as mentioned above, tends to reduce the ability of plant
pathogens to survive. In addition, a soil layer over plant debris provides a
physical barrier that prevents spores that are produced by fungal pathogens from
reaching new healthy plant tissue. Finally, in more rural areas, burning of
plant debris can help in eliminating plant pathogens, although this technique
can contribute to air pollution and is not the disposal method of choice.
Pruning
Late
fall and winter are the best times to prune many woody ornamentals (see UW
Garden Facts
XHT1013,
XHT1014 and
XHT1015). Pruning during colder weather, when pathogens are less active,
can help lessen the possibility that pruning wounds will become infected by
canker-causing fungi or bacteria, thus leading to a reduction in canker diseases
that could potentially girdle and kill branches or even entire trees. Late
fall and winter pruning of oak trees is critical in reducing the incidence of
oak wilt (see UW Garden Facts
X1075). If oak trees are pruned during the growing season, the sap from the
resultant pruning wounds is attractive to sap beetles that can transmit the oak
wilt fungus. Sap beetles are not active however during colder weather and thus
oak trees pruned in the late fall and winter cannot become inoculated by these
beetles.
Proper pruning of trees and shrubs can also be important in helping prevent
foliar diseases. Pruning often opens up the canopy of trees and shrubs and
allows for better air flow within the plant. During the growing season,
the increased air flow will reduce the humidity around foliage and thus lead to
more rapid drying of leaves. Lower humidity will reduce the likelihood
that powdery mildew (see UW Garden Facts
XHT1005) will develop. In addition, many other foliar pathogens require
long periods of “leaf wetness” (i.e., periods where there is a film of water on
the leaf surface) for their spores in germinate and infect. Thus anything
(including proper pruning) that shortens the length of time that leaves stay wet
is likely to reduce the severity of foliar diseases.
Watering
Oftentimes in the late winter, or even into the spring, conifers begin to
turn brown. This browning is a disorder called winter injury. Winter injury
results when conifers (especially yews) do not have enough internally stored
water for their needs over the winter. As daytime temperatures become warmer in
the late winter and early spring, conifer needles begin to naturally lose water
(a process called transpiration) as they attempt to grow. During the summer,
this lost water would be replaced by water taken up by the plant’s root system.
However, in the winter and early spring, soil temperatures are cold enough that
the plant’s root system is not functioning efficiently. Thus the amount of
water lost by needles is not replenished by the water taken up by the roots. As
a consequence the needles dehydrate and die.
The easiest way to prevent winter injury of conifers is to make sure your
evergreens are well watered into the fall. Established trees and shrubs need
about one inch of water per week. If Mother Nature does not cooperate, then you
should apply water at the drip line (or more extensively if possible) of your
conifers using a soaker hose. Conifers can be watered up until the time that
the ground freezes.
With just a little effort in the late fall and winter, you can have a big
impact on the health of your landscape ornamentals next spring and summer. So
get back into the gardening mode, and use the remaining snow-free days of 2004
to prepare your garden for a beautiful and healthy 2005.
– Brian Hudelson, Plant Disease Diagnostics Clinic, Dept. of Plant Pathology,
UW-Madison/Extension
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