Issue 11, July 7, 2015
Rain and Plants
Plants need water to grow, but too much water can be as bad as too little, and in some cases, maybe even more problematic. Several things occur when there's too much moisture for plants.
The biggest negative is the lack of oxygen around the roots, which need as much air as leaves. While not big photosynthetic powerhouses like leaves, roots do breathe, and it's important to have oxygen present. When a plant receives too much rain or is overwatered, the large soil pores, called macropores, fill with water, preventing roots from exchanging gases.
Pines in flooded pond.
Typically, the macropores drain quickly, usually before the smaller micropores. However, if there is nowhere for the water to go, the pores stay filled and roots start suffocating and dying. Of course, without roots, the plants start wilting, which is rather ironic considering the cause.
Some plants are adapted to soggy soils. Many of the bottomland plants including alders, birches, pin oak, swamp white oak, silver maple, and many of the poplars are adapted to flooded conditions and aren't adversely affected. It's the upland plants that suffer. The more a plant can tolerate drier conditions, the less likely it's adapted to excess moisture.
Soils that drain quickly such as sandy ones have fewer problems. Heavy clay soils, which typically is what's left in an urban environment, have fewer macropores, and thus retain more moisture, leading to more plant death.
Excess moisture can also lead to disease such as Phytophthora Root Rot, one of the leading causes of rhododendron death. In poorly drained soils, the fungal pathogen multiplies quickly, causing roots to turn black. Stems and crowns may also develop black streaking. Plants immediately start wilting, which leads many homeowners to water more. Typically once a plant is infected with Phytophthora, it's next to impossible to control.
Excess moisture also keeps roots from seeking deeper soil levels. As stated above, the roots need oxygen, and stay closer to the soil surface. This becomes problematic for two reasons.
First, when it does get hot and dry, roots dry out quickly as the top inches of the soil dry out faster due to the heat. With some heat stress, they will eventually start heading deeper, but maybe not fast enough to avoid wilting. Getting a deeper root system helps plants overcome excessive heat and dry conditions.
This is the reason horticulturists promote long infrequent waterings instead of a little bit every day. Roots sense deeper, cooler, and moister soils and grow down. If the soil is constantly moist in the top few inches, the roots will stay there.
Second, without deep roots, the plant doesn't have as much structural support. Corn needs bracer roots in order to support itself once the ears start filling out and the plant becomes top heavy. Trees need the same. Without deeper roots to hold the plant erect, strong winds (or ice and snow storms this coming winter) can topple a plant easily. Staking may help initially and should be considered, but after a season's worth of growth, staking provides little benefit.
Increasing drainage by positioning downspouts away from sensitive plants, installing French drains to channel water away from plants, and planting sensitive plants on berms or in raised beds may help. (David Robson)
Author:
David Robson