This week our SoCal Garden Clinic turns to problematic avocado trees:
Question from reader Steven Klein of Malibu: In November 2011, I planted a 3-gallon Lamb Hass avocado tree on a slope with full sun about 90% of the day. Despite my ineptitude, this tree continues to survive, although it has lost several branches and 65% of its leaves. There is some sign of new growth, and I would like to help it along, even though it may take several years.
A similar mature avocado tree existed in the same general location for years and did well with almost no care until it was consumed by fire. Any advice?
Answer from Frank McDonough, botanist at the Los Angeles County Arboretum and Botanic Garden in Arcadia: Although avocado trees can be planted in some pretty poor soils, they still need very good drainage. In a follow-up conversation, you mentioned that you planted your avocado tree in adobe soil in a hole that was amended with 50% organic compost. This huge amount of water-absorbent organic material in the planting hole soil creates a wicking effect, and the water in the surrounding soil migrates into the planting hole, keeping it moist.
Now add your watering schedule — every two to three days — and what you have is a pond with a struggling avocado tree in the middle of it. Soggy soil is a perfect breeding ground for an organism that is the bane of avocado growers: phytophthora. Once thought a fungus but now recognized as a relative of kelp, phytophthora’s spores have a tail that allows them to swim through water toward the roots of susceptible trees, so whenever you have standing water between soil particles, you have a swimming hole for phytophthora and, eventually, a dead avocado tree.
Because the tree is showing new growth, it might be tempting to invest more effort in nursing it along. That’s a bad idea. It’s better to start over.
Plant a new tree in a hole about two to three times wider than the container in which the tree came, and just as deep. Do not use any soil amendment, just the native soil.
Water regularly (once or twice a week) for the first few months, then after the tree is established (two to three years), water no more than a few times a month. Also remember to keep your avocado tree mulched with up to 4 inches of organic material placed under the canopy of the tree, taking care not to pile it up against the trunk.
Do not fertilize your avocado tree for at least a year. Some fertilizers, if applied too soon after transplanting, can burn the roots of the young tree, causing irreparable damage.
If you do decide to nurse the existing tree, treat it with the fungicide Agri-fos, so the phytophthora gets eliminated.
Question from reader Lynne Winner of Castaic: We have a Fuerte avocado tree that needs to stay dwarf size although it is a regular tree. I recently pruned the top but left the rest of the tree untouched. In the five years we have had this tree, it has produced a total of four avocados. My husband thinks that pruning it causes the tree to go into shock and is the reason why it bears minimal fruit. Is it harmful to prune an avocado tree? Why do we get so few avocados on our tree?
Answer from McDonough: Avocado trees rely on their thick canopy of leaves to protect their thin, green bark. Topping an avocado tree can expose tender inner branches to sunburn. Damaged trees under a lot of stress will drop fruit. The tree can be protected by whitewashing the trunk and major limbs with white latex interior paint diluted 50% with water.
But pruning isn’t necessarily the issue here. Most parts of Castaic are in a climate zone too windy for most avocados. The wind causes the avocado trees to drop their blossoms, and so you’ll have a hard time getting any fruit. If you can shelter the tree from the wind without cutting sunlight, you might get some fruit from the tree.
We welcome reader questions about what to plant and how to solve your gardening conundrums. Sent inquiries to firstname.lastname@example.org with “SoCal Garden Clinic” in the subject line. Because of the volume of mail we receive, we can respond only to select questions