How Often to Water Trees in the Desert/Possible Iron Deficiency - Knowledgebase Question

Tempe, AZ
Question by ImPNeary
June 18, 1999
Last fall we had two Brazilian Pepper Trees planted in the front yard - lots of sun. Both doing well, one doing extremely well, until recently lots of the leaves turning yellow. We have been told possible iron deficiency, then under watering. How often and how long should we water - its over 100 deg.

Answer from NGA
June 18, 1999


Underwatering, overwatering and iron deficiency are all possible explanations. Always water slowly, deeply and infrequently. If you remember nothing else, this method will help your plants thrive.

Desert soil and water both contain salts, which can accumulate in the root zone over time. This salt buildup forms where the water stops penetrating. If you ?sprinkle? plants lightly and frequently, salts will build up in the top layers of soil and damage or kill your plant. Deep watering?or leaching?prevents this by flushing the salts past the root zone.

Roots also need oxygen to survive and soil that is continually wet doesn?t provide it. Use a soil probe (any long, pointed piece of metal or wood to poke into the soil) to check how far water has penetrated. The probe moves easily through moist soil, but stops when it hits hard dry soil. For trees, water should reach about 2-3 feet deep. There are numerous variables involved for watering schedules, such as type of soil, how fast or slow it drains, sun and wind exposure at your site, temperature, age and condition of the plants and much more. It?s important to learn the specific needs of your landscape, both for its health and your water bill. Use the information above to determine how moist the soil is before automatically applying more water. The following frequencies are guidelines only for the summer months when temperatures are high (100+) and winds are drying.

Trees, 1st year once a week
Trees, 2-5 years every 10 days
Trees, 6+ years once a month

Iron chlorosis is also a problem in the desert. Overwatering, combined with heavy summer rains, can promote root rot, fungal diseases and iron chlorosis. Chlorosis is recognized by new leaves that are yellow, while the veins remain green. If the condition is severe, the entire leaf may be yellow. Queen palms, pyracantha, bottle brush and silk oak trees often suffer from chlorosis.
Although iron may be present in the soil, it is not always in a form that plants can use. Overly wet soils are depleted of oxygen. (As water fills in the minute spaces between soil particles, air moves out.) Plant roots need oxygen to absorb iron in the soil. To help prevent chlorosis, always water slowly, deeply and infrequently.
Soil with a high pH (alkalinity) also inhibits iron absorption. If you are using correct irrigation methods and symptoms are still present, apply iron chelates or ferrous sulphate to the soil. Both are readily absorbed by a plant?s roots. I hope this information helps!

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