Overwatering vs. Underwatering an Avocado

Root Spread

• When grown in preferable conditions -- a full-sunlight location with well-drained soil, an avocado tree produces a root system that does not mirror its canopy. The tree grows shallow roots that extend slightly beyond its drip line, which is below the tips of its outermost branches. With its roots remaining in the top 6 to 8 inches of soil, the tree needs light rains or short irrigation sessions for moisture uptake. For example, your avocado tree should need up to 20 gallons of water each watering session. During hot weather, the irrigation may be necessary several times each week to maintain moist soil conditions. Avoiding underwatering and overwatering is critical for the tree's healthy fruiting and root growth.

Waterlogged Roots

• Overwatering an avocado tree affects its roots initially, and the water's effects slowly spread to the leaves. Waterlogged shallow roots begin to decay and turn black. Because the roots slowly stop absorbing nutrients and moisture, leaves wilt and turn brown. Avocado root root is caused by the pathogen Phytophthora cinnamomi. Avoid it in your tree by allowing its soil to dry between watering sessions. Use a drip-irrigation system, which waters at the soil level, rather than soaking the area with an overhead sprinkler. Less evaporation occurs with a drip-irrigation system, allowing soil to absorb most of the provided water and the tree's roots access to that water.

Drought-Stressed Roots

• In contrast, an avocado tree that is under drought stress often drops fruits and has stunted growth. Do not plant your avocado tree in lawn; keep all competitive plants away from its trunk. Spread a 4- to 6-inch-thick layer of mulch on the ground under the tree's canopy, but don't allow the mulch to touch the tree's trunk or bark rot could result. Mulch conserves soil moisture, which is especially important when warm weather arrives and irrigation session frequencies are not increased. Soil microorganism activity increases with mulch use, and the microorganisms combat Phytophthora cinnamomi. If you want to increase soil moisture reserves beyond mulch's capabilities, irrigate the site in early morning or evening. Sunlight is minimal at those times, which decreases the amount of water lost to evaporation.

Moisture Sensor Instruments

• Moisture sensor instruments record the amount of water in soil, and using them helps to time irrigation sessions correctly. The sensor portion of the instruments needs to be covered completely with soil; no air gaps can be present. Firming soil against the sensors allows them to provide their best soil moisture readings.