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Originally Published November 2010.
Welcome to fall! On this side of summer, we have just come through a pretty rough drought (as of October 19 we were in the Severe – Extreme range) and your trees responded to that lack of water. Of course, we’re not necessarily out of the drought just yet, but hopefully we will be soon. What happened? What can we do about it? Is there hope? Read more, We’ll tell you, and Yes.
In response to drought, or a soil water deficit, one major response of trees is to slow or decrease growth in the root zone. This in turn causes the root zone of the tree to decrease in size which means the structural holding capacity of the tree is then reduced.
That’s all well and good except that when it rains following this drought, the ground will, at some point, saturate with water and the wind will blow. With a decreased root zone, the capabilities of the tree to hold themselves in the already saturated ground may not be enough to hold them upright in heavy or prolonged winds.
In addition to a decreased root zone, low water levels can lead to Armillaria root rot, sometimes called “shoestring root rot” because of shoestring-like structures (rhizomorphs) the fungus forms that invade root systems. Armillaria is an opportunistic fungus, or secondary fungus.
It is generally present in soil and on the surface of roots but only invades when roots are weakened or damaged by stresses, such as drought. In a healthy plant, the disease can be isolated or compartmentalized through natural processes. A drought stressed tree with reduced metabolic activity, however, may not be able to properly compartmentalize the disease. The fungus invades cambial tissue and is capable of killing smaller trees in a season or two. Large trees with an extensive root system may not begin to show symptoms of decline until large portions of the root system are killed by the fungus. This may occur up to several years after the drought event and initial invasion by the Armillaria fungus. There are no chemical treatments for Armillaria. The best treatment is prevention.
The thing about trees is that rarely is it Just One Thing that you can pin point that kills a tree. Okay, yes, a lightning strike could do it. We are not talking about that right now. Typically it is the compounding of issues that add up to a tree’s decline and death. As in the above situation where a lack of water causes the decline of the root zone, the decreased ability of the root zone of the tree leads to a metabolically weakened tree, which in turn makes it easier for opportunistic fungi (like Armillaria) to infest a root system. This leads to an even more radically declining root system, which leads to less metabolic activity which means more pests infest the tree and then a tuant coup or killing blow. Sometimes it comes as a heavy wind storm, sometimes a lightning strike, a fire. Before you know it, a section of fibers fail and the tree falls to the ground.
All of this happens gradually over time. A drought one summer, a mild and dry winter, a decent spring, another droughty summer, a wet fall and then heavy winter storms; year after year for 4 or 5 years in a row. In the 5th summer there’s a heavy storm with lots of rain and wind, maybe some hail, but definitely heavy rains that soak the ground and heavy winds that blow and rock the tree. You see where this goes. Some of those trees do survive the storm. They make it on through the next issue, but there is a problem. As the tree has survived in these past years it has been using resource stores that it has built over its life and has relied on primarily for getting going in the spring. But this spring it flourished, put out leaves, and then just died. The tree did not have enough stored resources to overcome the deficits that had built up over time.
Short answer: Yes. Long answer: Yes with a “but”.
Here is the situation: Mother Nature is a great builder and she builds amazing things like mountains and canyons and trees. Typically, the bigger something is, the longer it takes to get there. This goes the same with fixing big things. It’s not something that happens overnight. Years of decline can only be reversed through years of active maintenance and care.
The best methods of doing this are two really simple steps: 1) Watering and 2) Mulching
1) Watering (See Tree Tip from August 2008)
• Do not water the area directly against the trunk. This increases the risk of diseases and rot. Roots extend far beyond the edge of canopy or drip line. Watering the outer half of the area under the canopy and beyond the edge of the canopy will put the majority of the water where the majority of the fine feeder roots are.
• Water deeply rather than frequently. Because most tree roots are found in the upper 18 - 24 inches of the soil, this is the zone that should be wetted up in each irrigation cycle.
• Stop watering when runoff starts. Soils high in clay content accept water slowly. Water infiltration is especially slow in compacted soils. If water starts to pool or run off, stop watering, let the water soak in, and then begin watering again. Ideally, you want to repeat these “on/off” cycles until you apply enough water to wet the soil to 18-24 inches. This may take a number of cycles over several consecutive days. A good rule of thumb would be 15-20 minutes, 3-5 times a week.
• Don't saturate the soil for long periods. Water displaces air in the soil; so long periods of soil saturation can suffocate growing roots (i.e. drowning). Take a long enough break between irrigation cycles to allow the free water to be absorbed. If in doubt, probe or dig to make sure that the soil is not getting soggy below the surface. Again: 15-20 minutes, 3-5 times a week.
2) Mulching (See Tree Tip from October 2010, or March 2009)
• Pick an appropriate time to mulch. Mulch annually in spring before soil moisture decreases and temperatures increase or in late summer/early fall. Mulch Prior to, during and after construction or infrastructure changes affecting tree roots and tree health. Mulch after tree injury. Or mulch because you feel like it.
• Find and a good quality mulching material. Organic materials are preferable to inorganic materials but when organic mulching materials decompose, they must be replenished. I recommend these mulches: Wood chips, pine needles, tree bark, leaf mold or compost.
• Always use this mulching method: Apply mulch in a circle covering the entire root system of a tree. Most of the fine, absorbing roots of a tree extend well beyond the tree canopy, or drip line. The general recommended mulching depth is 2-4 inches.
• Want to read more about trees? We have quite a Tree Tip library. Feel free to browse through and read up on our favorite subject.
• Make sure you are not mulching with a deep layer of material over six inches. Or mulching with piles high against the trunks of young trees. This can lead to insect and disease problems. Keep mulch off from the base of the tree trunk to help reduce the chances of crown rot and pest infestation.
• Think about how mulching benefits the tree by: Conserving moisture · Improving soil structure · Reducing soil compaction · Increasing soil aeration · Increasing the available nutrients · Suppressing grasses and weeds · Making the arborscape attractive · It helps prevent damage from mowers and weed whackers
One last item: Summer is not the only season where droughts can occur. Winter is a perfect and more common drought time. Long periods of cool or cold weather with no precipitation, low humidity levels and wind sucking more moisture out of the soil. Just because there are no leaves on the trees during winter, it does not mean that your trees stop growing. They just don’t grow as fast or as noticeably, but they are still growing. Keep watering during the winter, maybe just not as frequently.
Of course, we at Woodland Tree Service hope that your trees are healthy, doing well, and you are enjoying them. If any of these things are not happening, we would be happy to come see how we could help and realign those three items. A healthy tree leads to a healthier landscape and a happier homeowner.
We want you to be that happy homeowner. If you have any questions or need any assistance with your trees, pests, or just questions, contact the experts at Woodland Tree Service. Email us or call (901) 309-6779.