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PLANTS SUFFER LEAF SCORCH FROM HOT WEATHER

written by

Marianne C. Ophardt

WSU Extension Faculty

for the Tri-City Herald, Kennewick, WA

Days and weeks of intense summer heat and sun in our area cause trees, shrubs and other plants to loose large amounts of water through the pores, called “stomata,” in their leaves. This loss of water from the leaves is a process called transpiration.

The purpose of transpiration is to keep plant leaves from cooking in the sun. It’s a plant’s natural method of keeping cool. At least 95 per cent of all water absorbed by a plant’s roots is used for transpiration and cooling plant leaves. Water is also used by plants for normal cell function and growth. In addition, roots absorb water containing nutrients needed for plant growth.

When not enough water reaches the leaves, the leaf tissues become “burned” or scorched. Leaf scorch is a common problem that arises during our area’s hot, dry summer months. It appears as brown to tan, dry leaf edges. This necrotic tissue may also extend into the areas between the leaf veins. On needled evergreens, needles will turn reddish purple first and then brown, especially on spruce.

Leaf scorch will typically be more severe on the south to southwest side of a plant or on the side more exposed to sun, heat and wind. When a tree or shrub exhibits leaf scorch, it’s an indication that not enough water is getting to the leaves. The most logical cause of leaf scorch is a lack of adequate soil moisture. If leaf scorch starts showing up on a tree or plant in your yard, check the soil moisture first.

For large shade trees and shrubs situated in lawns, the 15 to 20 minutes of irrigation applied every day or every other day to keep turf green is simply not enough. These larger plants need a deep watering once a week during hot summer weather. How deep is deep? A “deep” watering for a tree only needs to moisten the soil to a depth of 12 to 18 inches. That’s where most of the water absorbing roots of trees are located. For other garden plants check the soil and make sure it’s moderately moist. The exception is a xeric or low-water-use landscape with drought tolerant plants. These plants can withstand drier soil once established.

A lack of adequate soil moisture isn’t always the cause of leaf scorch. There can be underlying causes preventing water from entering the plant or keeping water from reaching the leaves. These causes include:

1. Inadequate root systems that are unable to absorb enough water to provide for the plant’s needs. This may be due to a lack of room for root growth, damage to roots from construction activities or cultivation, or compacted soil that impedes root growth.

2. Injuries to tree trunk or plant stems can prevent the upward movement of water to the top of the plant, even if the roots are healthy and able to absorb adequate amounts of water.

3. Girdling or restricted roots “choke” a plant and interfere with both the absorption and movement of water to the top of a plant.

It’s important to point out that certain plants aren’t well adapted to our hot summer conditions and are prone to drought stress and leaf scorch. Plants like flowering dogwood, Japanese maple, blueberry, and rhododendron do best if planted where they’re protected from hot afternoon sun. It also helps to keep their roots cool by mulching with shredded bark or compost .

While moderate leaf scorch itself is not a fatal blow to a tree or most woody and perennial plants, the deficit of water and the damaging stress it represents is serious and can lead to a plant’s decline and eventual demise. If a tree or plant in your yard or garden starts showing signs of leaf scorch, you should try to determine the cause and take immediate steps to rectify the situation. Don’t let your plants continue to suffer in the heat.

Published: 8/11/2007 2:38 PM

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