1 - Monsoon rains may not provide enough moisture to replace irrigations, it depends on how deeply the soil is moistened.
Yard and Garden July 6, 2013
Q.
If the monsoon rains really come this year how much rain will it take so that I will not need to irrigate?
K.M.
Albuquerque
A.
From the time you send this question until I wrote the answer, some rains have come. I have measured 0.02 inches in two rain events at my house. Other areas nearby received only a trace of precipitation and other areas got more rain. In no case was there enough to replace even a single irrigation event. The amount of rainfall in a single precipitation event is not the important factor. The amount of moisture that soaks into the soil is what is important. A slow rain over a period of several days would, perhaps allow you to skip irrigating as long as it was raining. For garden flowers and vegetables, the soil should be moistened to a depth of a foot. Trees and shrubs need soil moistened to a depth of 2 to 3 feet depending on their root systems. Probing the soil with a metal rod or carefully digging to see how deeply the soil was moistened by rain will allow you to determine if you can skip irrigations.
Our monsoon rains rarely come as slow, steady rains that can soak into the ground. These summer rains often come as downpours associated with thunderstorms. This sudden rain often runs off instead of soaking into the soil. Gardens with swales to retard runoff and direct water into the soil will receive the greatest benefit from our hard rains. Proper landscape design and grading can help maximize the beneficial effects of monsoon rains, but even in properly designed landscape the gardener should be prepared to irrigate within days after the rain events. Water harvesting, directing roof runoff into appropriate areas of the landscape will also help minimize irrigation.
Landscape plant selection can also make a difference. Those plants able to survive drying between irrigations will probably make the most efficient use of monsoon rains and allow you to reduce irrigation and use of purchased water.
Properly prepared soil areas are also helpful in maximizing the benefits of monsoon rains. Compacted, dry soils often shed rain and increase loss to runoff. Loosened soil, moist soil, and mulches may help the soil accept the rain. Organic mulches are very good in most garden situations, but may intercept moisture before it can reach the soil. In areas where you use organic mulch, check carefully to see if the soil under the mulch received any moisture and how deeply it was moistened.
Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.
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