Trees and Heavy, Continuous Rainfall

We have recently been inundated with multiple events of heavy rainfall – a whole lot of water in a short amount of time. While rain is a welcome relief for landscapes in the hot, dry summers of Oklahoma, there can be a little too much of a good thing, which can cause some issues for young and mature trees alike, such as:

General Tree Stress


Trees respire through their roots, which mean that they take in oxygen and release carbon dioxide through air spaces in the soil. When soil becomes saturated and those pores are filled with water, this function is disabled which can cause stress for the tree. There’s not a lot that we can do about trees affected by standing water after a big rain as it’s happening, but it’s important to try to prevent your tree from becoming more stressed after conditions improve to avoid continued problems.
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  • Consider adding a 3-4” thick layer of mulch out to the drip line of the tree.
  • Give the tree a long, deep soaking later in the season when it gets hot and it hasn’t rained in a while.
  • Turn off your lawn irrigation system during weeks when it has rained to let the soil dry out a little. If the soil is wet, the grass doesn’t need to be watered. This can reduce your water bill too!
  • Do not apply fertilizer when your tree is stressed unless a soil test shows a mineral deficiency, then only amend for that specific problem. Unneeded fertilization stimulates growth that a tree must then use energy to support, further stressing an already weak tree.
  • Finally, flood stress can be avoided entirely by not placing trees in locations that tend to hold water, or by using flood tolerant species.
Most trees can tolerate short durations of saturated soil conditions when they have a chance to dry out and recover.

Defoliation


A more specific symptom of saturated soil conditions shows up in the leaves. Foliage may start to turn yellow and then fall off, particular in the lower, inner part of the canopy. If soil is allowed to dry out, trees will usually put out a new flush of foliage growth, but prolonged defoliation contributes to tree stress.

Other Foliage Problems


Cool, humid conditions are perfect for fungal diseases to flourish in. We tend to see a lot of leaf spot and powdery mildew in years with a lot of rain. These problems are usually just cosmetic and will not affect the tree long term, but they can cause minor stress. If a tree defoliates, it’s likely to produce new foliage. Just work at keeping the tree healthy otherwise (see the suggestions above), and remove any fallen leaves to dispose of so they do not re-infect healthy foliage. Make sure your irrigation does not splash leaves, which can spread spores and prolong infection. If it’s raining this much, the landscape probably does not need to be watered anyway, right? Repeated defoliation over multiple seasons can cause greater stress and result in reduced growth and additional pest and disease problems. Fungicides may be necessary in these instances, applied early in the season, before symptoms occur.

Root Problems


We have had some issues this spring with trees planted in the last few years having their roots loosened so that the tree has an unnatural amount of sway. This compromises the anchorage of a young tree and can lead to further stress. We recommend placing stakes to support the tree while its roots re-establish in the surrounding soil, to be removed no more than one year later. Larger trees can also experience root issues sometimes after heavy storms/rain events. Signs of root failure include mounding of soil on the side opposite a lean that is becoming more pronounced. If you notice that one of your trees is experiencing this problem, it’s a good idea to contact a Certified Arborist for a tree risk assessment. Saturated soil conditions are also conducive for root rot organisms. Fungal fruiting bodies at the base of the trunk or near roots could be indicative of root decay. Tree roots scoured out by eroding soil can make a tree more susceptible to root failure.

It’s a good idea to go outside and take a look at trees in your yard after big weather events, just to ensure there are no issues that need attention from a Certified Arborist. Taking steps to maintain tree health throughout the year can also potentially reduce the adverse effects experienced from saturated soil conditions.


View other topics from the Summer 2015 edition of Edmond Tree Mail

Check out last season's tree issue article