Home & Garden: How to combat winter drought
Free Press Gardening Columnist
As I write this week’s column on Monday morning, Feb. 23, the snow is still coming down — and I am still hoping for more.
After it snows in Colorado’s Grand Valley, it will melt and soak into the ground to replace moisture we lost over this winter through evaporation. Without snow melting into water, which can sink into the ground, our landscapes and fields will likely need a drink. Hopefully water comes in the ditch early this year so we can replenish this winter’s soil-moisture deficit.
Soil moisture is only one of the environmental stresses our plants have to contend with in our dry, desert environment. Too much moisture, which occurred in the floods last spring on the Front Range, is as damaging (if not more so) as too little soil moisture.
Plant growth — like dry matter accumulation — is directly related to the amount of water plants transpire. Transpiration is the process where water is taken in by the roots, moves through the plant’s vascular system, and escapes as vapor from the stomates, the breathing pores on the leaves. When we have a water deficit (or excess), we have restricted root growth, restricted water uptake necessary for movement of nutrients into the plant, and reduced plant growth and yield.
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Effects of moisture deficit stress reduce the amount of calcium and other nutrients moving into the root system. As roots begin to dry out due to lack of soil moisture, they form a waxy coating around their vascular bundle called the casparian strip. The vascular bundle consists of the xylem tissue necessary for movement of water and nutrients up the plant. The vascular bundle also contains the phloem cells necessary to move sugars, starches, and hormones down the plant from the leaves and buds.
The waxy cylinder forming around the vascular bundle is a natural maturation process of roots. When this happens prematurely, as it does with soil moisture deficits, it greatly affects the ability of the plant to continue proper growth. This soil moisture deficits also results in the loss of root hairs and mycorrhizal-forming fungi. While not all plants have rooted hairs, those that do use these root hairs to significantly increase moisture and nutrients. The mycorrhizal-forming fungi act in a symbiotic way to increase water and nutrient uptake of plants while they use some of the plant’s carbohydrates. Dry soil results in the loss of root hairs and the death or even shifting of mycorrhiza from a symbiont to a parasite. In its parasitic stage the mycorrhiza increases root death by removing carbohydrates from the plant without contributing to the plant’s growth. This further weakens the plant increasing herbivore feeding.
Plants subjected to moisture stress increase the concentration of solutes in their cells. One of these solutes is an amino acid called proline. Proline has been shown to stimulate the feeding activity of insects. As soil moisture stress continues you can expect more aphids, mites, and even Mexican bean beetles feeding on your vegetables. Spider mite injury on corn, fruit trees, and ornamentals of all types can be severe when there is inadequate soil moisture.
Now you may think you can correct this problem when irrigation water is available in spring, but it takes plants a long time to rebuild roots that are damaged by winter drought. A good example of this is the white birch. White birch can lose most of their absorbing roots due to winter drought, yet you may not know this has happened until this tree start to die back from its branch tips in July or August due to an in adequate supply of root-absorbed moisture. So even after four or five months, assuming the root system starts to develop new replacement roots in April, the tree has not replaced a sufficient quantity of those roots to provide the water needs of this tree in the hot months of summer.
Plants naturally produce organic compounds to defend themselves from insects, mites, and pathogens. In order to produce these compounds, plants require nitrogen. Moisture stress reduces nitrogen uptake as does the loss of roots due to winter drought. Consequently winter moisture deficits directly and indirectly impacts the plant’s long-term health. Hopefully we will receive sufficient moisture from this storm to help improve the moisture content in our soils. Otherwise we should look for an increase in insect and mite problems this coming summer.
Free Press columnist Dr. Curtis E. Swift is a retired horticulture agent with the Colorado State University Extension. Reach him at Curtis.Swift@alumni.colostate.edu, 970-778-7866 or check out his blog at http://SwiftsGardeningBlog.blogspot.com. He owns Swift Horticultural Consulting and High Altitude Lavender.
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