Most commercial avocado orchards in Hawaii are located where they receive an annual rainfall of 50 to 80 inches, which is optimal for growth and fruit production. Avocados grown in drier coastal areas produce fruit earlier, but require supplemental irrigation. Avocados must have regular irrigation during their flowering and fruit set stage or the fruit may drop or shrivel due to drought stress. Too much irrigation can cause waterlogged soils and lead to Phytopora root rot, a major fungal pest in Hawaii. Proper irrigation and attention to drainage is essential to growing healthy trees and harvesting delicious avocados.
Examine the soil in which the avocado is planted. Porous soils are best for optimum health, but may require more frequent watering in drought, especially during fruit set. If the soil is heavy, carefully work in up to one cubic yard of black cinder and one cubic yard of well rotted organic compost per two cubic yards of soil around the outer edges of the tree canopy.
Create shallow irrigation wells, six to eight inches deep, at the outer limits of each tree's canopy so that irrigation water does not run off. Slope the soil so that the area around the trunk is a few inches higher than the outer ring of the well.
Apply a six inch layer of organic mulch inside the irrigation well, carefully avoiding the area one foot out from the tree's trunk. Maintain this layer of mulch to mitigate moisture loss.
Fill each well with enough water to submerge the outer ring but leave the trunk just above the water line. Irrigate your avocados up to once a week in dry times, as necessary to maintain evenly moist soil and prevent the young leaves from wilting. Halt irrigation completely during wet weather to reduce the risk of fungal disease.
Test the soil moisture during the flowering and early fruit period by inserting a dry, ½ inch diameter, wooden dowel three inches into the soil in the tree well, avoiding the roots. Pull out the dowel and check if the end is moist. Water your trees thoroughly when the dowel is dry.