The last significant rain we had in the Louisville, Kentucky, area was on June 16, 2019. On that day, 1.89” of precipitation was recorded1. The rain came down softly, thoroughly saturating the ground and perfectly suited for our trees’ root systems to utilize. Prior to June 16, we were in a surplus, with +11” above average for the year.
Last year, by December, we had amassed +20” above our average in precipitation, mostly due to the massive amount delivered in September—12.12”—a month when average rainfall just measures 3.03”. This followed an above average amount of rainfall in the months of June and July, 6.67” and 5.07” respectively, which triggered many of our trees and shrubs to set their fruiting and flowering crops for 2019 into overdrive. We have been observing this with the abundance of fruit–peaches and pears, to name two–and the large number of cones and seeds on our conifers, oaks, and maples.
This spring, following a mild February and again above average precipitation—10.47”—our trees and shrubs fairly came out of dormancy bursting with color, and an almost tropical-type lush foliage. We continued to enjoy a lovely spring, into May and June, and then… The faucet turned off. July recorded precipitation was 1.48”, August a little bit better at 2.68”—not really—but September, notably and historically Kentucky’s driest month, barely registered a trace at 0.04”.
Results of a dry, hot three months? All of that new, lush growth has either gone dormant or died and those plants that were experiencing any type of stress have declined to the point of no return. Our state tree, the Tulip Poplar, and other trees sporting large leaves have been losing their foliage since mid-July. Fungal and bacterial diseases that lie dormant in healthy plants and surrounding grounds are now making an appearance in the forms of wilt, scorch, canker, and blight. Trees that normally are approaching dormancy in September due to the shortening of daylight are holding onto their foliage in October, fully green and fully leafed out.
For those trees that are standalone specimens in our residential landscapes, if they are not being regularly watered, prognosis for their continued good health over the next 2-5 years is an uncertainty. The small bit of water received of those with irrigated lawns will help somewhat, but normally irrigation runs just long enough to wet the grass and its root system and is never left on to ensure a deeper saturation for the trees’ diverse root system, which amasses and spreads out further from the trunk and at an average depth of 6”-18”. A healthy tree that stands 30’ tall can potentially have its root system encompass 90’ in diameter. Those outer roots are the workhorses, taking in more than 40% of available water and nutrients. And, as the soil becomes drier and starts to crack, these roots begin to dry and die back, too. Without these roots, next year’s growth and health are indeterminate.
To help replenish a tree’s stored supply of starches and soluble sugars that were most likely used up during the drought, applying a water-soluble, subsurface fertilizer is recommended. This can be done during the dormant season, as the roots will remain active during our late fall and winter months. Regrowth of the fine outer roots is key to maintaining a tree’s ability to counteract adverse weather conditions; i.e., winter and spring storms and further damage from future droughts. It is important to note that the addition of mycorrhizal fungi into your fertilizing program will aid in the roots’ ability to transport water and to move through the soil. Mycorrhizae have also been shown to aid in the roots’ ability to uptake nutrients, thereby reducing stress in our trees2.
In summary, drought conditions do have an adverse effect on our trees. The longer we remain precipitation-free, the more damage is being done. We can help to alleviate this by initiating and maintaining a regular watering program, done slowly and over a longer time period, and by replenishing a tree’s food supply. Without these external aids, a drought-stricken tree, having used up its stored starches and sugar supplies, becomes susceptible to damaging diseases and insects. These, in turn, will contribute to its decline and eventual death.