Yes, it happens and it can kill trees and shrubs. The prescription? Irrigation.
- When?
- Sooner rather than later.
- And please include street trees while you're at it (between sidewalk & curb).
- Why?
- Lack of water reduces a tree's ability to manage heat stress and conduct photosynthesis, weakening it and limiting future growth.
- Drought stress increases the tree's susceptibility to insect and disease pests that would not typically affect healthy, non-stressed plants.
- Where?
- For young trees (planted within the last 5 years or so)
- Water directly over the original root ball.
- Water bags are excellent for this and are available at Forrest Lytle on Galbraith Rd.
- For other trees, water under the canopy or within the dripline
- This is the area of ground which is shaded at high noon.
- A sprinkler or coiled soaker hose will do.
- Dose, frequency?
- During normal droughty periods, -- 1.0 - 1.5" / week.
- For small and medium trees (up to 7" diameter or 8 - 15" diameter), that's about 5 gallons / inch of trunk diameter every 5 - 7 days.
- Under the extreme heat and drought we're currently facing, 2.0 - 2.5" / week would be better.
- For those small to medium sized trees, bump the volume up to 10 gallons / inch of trunk diameter every 5 - 7 days.
- Healthy, large trees have the reserves to tolerate normal, seasonal drought conditions.
- To keep them at their best, some irrigation is a good idea, but is not needed quite as frequently as the younger trees.
- Figure on applying about 750 gallons of water per 1000 square feet of area under the canopy.
- A square with sides 30 feet long = 900 square feet.
- If you're pacing this off, average stride length =
- 2.2' for women (13 - 14 strides = 30')
- 2.5' for men (12 strides = 30')
- Ancient trees are more like young trees.
- Denying them water is like daring them to live.
- How long does my sprinkler need to run?
- It depends on your water pressure and the specific sprinkler you use.
- Here's how you figure it out:
- Get a 5 gallon bucket, your hose, sprinkler, and a timer which counts seconds.
- Attach your sprinkler to the hose you'll be using.
- Place the sprinkler in the bucket.
- Turn the water on.
- Time how long it takes to fill the bucket.
- Multiply trunk diameter (in inches) by the minutes it took to fill the bucket to get "normal" dry season watering volume.
- Double that for periods of extreme heat and drought.
- If you can't manage the 5 gallon bucket check, then run with a guesstimate of 10 gallons / 5 minutes
- This could be way off in some situations.
- Or, check the depth of soil moisture you achieve.
- An inch of water moistens soils in our area to a depth of 6 - 8". Use a spade or soil sampling tool to check (not recommended around very young trees).
- The majority of a tree's roots are in the top 12" of soil. So moistening soil to a depth greater than 10 inches is unnecessary and doesn't provide much benefit.
- Mulch will help conserve soil moisture and keep soil temps cool (well, cooler).
- Most "black" mulches become water-repellent during dry spells (like peat moss).
- If water is running off (soil will be dry underneath), cultivate the mulch thoroughly to break that hydrophobic cap.
- Additional info:
- Though written for a different climate zone, this is a pretty useful document -- http://www.coopext.colostate.edu/4dmg/Trees/caring.htm
Background on heat stress / stroke in woody plants
When daytime temperatures exceed 86F, plants begin to adjust biologic processes to reduce stress. As long as precipitation is sufficient, all but the weakest (young & old) can tolerate normal levels and duration of heat stress each year.
Extreme stress leads to heat stroke in woody plants. And even if a plant appears to survive, it's been forced to waste resources, rendering it more vulnerable to lesser stresses in the years to come.
Three factors affect whether heat stress becomes extreme.
- Duration -- when "low level" stress temperatures (86F - 89F) last for weeks.
- Degree -- when daytime temperatures exceed 90F for more than several days, and / or when nighttime temps don't fall below 70F.
- Drought -- when precipitation or irrigation are insufficient to allow the plant to marshal all defenses.
2010 was a perfect example of extended "low level" stress -- by and large, the daytime high temperatures stayed below 90F, but they persisted for weeks in the upper 80s, and the drought persisted for months. We are still seeing trees die from that extended period of water deficit and excess heat.
2011 is shaping up to hit trees with a trifecta of stresses.
- The unusually wet spring resulted in fatal root rots for some trees. Many of those that survived are exhibiting unusual levels of fungal diseases on leaves, limiting photosynthesis.
- Mid-summer has arrived, delivering dual challenges of extreme heat and drought.
For tree geeks that want more detail, here are a couple of articles from Kim Coder, PhD at the University of Georgia Warnell School of Forestry. Each is about 10 pages long.
Heat stress syndrome
Heat stroke in trees
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