Tuesday, June 19 , 2018, 6:48 pm | Fair 66º


John Schnackenberg: Caring for a Santa Barbara Classic — the Avocado Tree

Irrigation and mulching are key to keeping the trees healthy and productive

Avocado trees are hard to avoid in Santa Barbara County. If you stand almost anywhere outdoors and scan the landscape in 360 degrees, it’s almost impossible not to see one. They’re everywhere — from backyards to whole hillsides — and their abundance and beauty are part of what makes the county special.

Once established, they aren’t especially fussy trees either. But they do have one major cultural need that is often neglected and is a common problem in commercial orchards and backyard trees. I’m talking about watering.

Good water management is the single most important cultural practice in raising healthy, productive avocado trees. If they don’t have enough of it, nothing else matters, and too much can drown the trees or create a soil environment that favors disease. Avocados are at a disadvantage when it comes to efficient water uptake. Their feeder roots don’t have a lot of root hairs, so there is less area on the roots to take up water and nutrients. Plants with lots of root hairs are simply more efficient at taking up water and nutrients.


To help make up for this lack of abundant root hairs, avocados have developed a close relationship with soil microbes of all sorts that help them fight off disease, and absorb water and nutrients. Keeping their roots healthy is a key component of water management and mulching the trees, especially when they are young and not fully established.

Mulching helps keep the tree’s roots insulated from high temperatures and drying out, suppresses weeds and helps provide a good environment for beneficial microbes to thrive. To aid beneficial soil microbes and young or new trees, evenly spread a shovel full or two of gypsum around the base of the tree and then spread a 2- to 4-inch layer of mulch over the gypsum, avoiding having it mound up against the tree trunk. For established trees, simply allowing the leaf mulch to build up (leaf litter layers of 1 foot or more is common in healthy, productive orchards) is sufficient without adding outside sources of it.

I cringe when people tell me they rake the fallen leaves from under their avocado trees. If you’ve done this, do your trees a favor and either put them back or spread some mulch under your tree to replace this loss. And avoid manure-based mulches for avocados because of high salt content. Avocado trees are very sensitive to salts, especially when they are young.


How much water to give your tree? The correct answer is, of course, enough. By this I mean enough water for the tree to thrive but not so much that you drown it or create conditions favorable for disease to develop.

Generally for older, established trees, irrigate them somewhere between one to four times per month. Irrigate more often during hot, dry, windy weather and less often during cool, cloudy, calm humid weather. Make sure you use at least enough water to wet the top 8 to 12 inches of soil. This is where 80 percent to 90 percent of the trees’ critical feeder roots are located. Depending on the size and age of your tree, this can mean using up to several hundred gallons per irrigation per tree for large mature trees. The bulk of the water should be applied at or near the drip line (a band several feet wide with the tree’s outer canopy margin as the center line).

Generally for young, newly planted avocado trees, irrigate two or three times per week during cool, calm, humid weather to daily during hot, windy, low-humidity weather conditions. For newly planted trees, this may just be a few gallons per irrigation. For young trees, water placement is critical and you should make sure the bulk of the water gets applied to the root ball. Just remember that a young tree has an inefficient root system and has fewer roots to take up the water. They can also drown easily if over-watered. These are general guidelines and not a substitute for getting outside and checking you tree’s soil moisture level.

The most straightforward way to tell how deep you’ve irrigated is to take a small headed or trenching shovel and dig in the tree’s irrigated zone. When the soil is wet to a depth of 12 inches, turn off the water. In sandy soils (relatively rare in Santa Barbara County), this means water more frequently with less water since in sandy soils the water tends to trickle down below the roots zone fairly quickly. For most of us with our heavier clay soils, irrigate less often with a bit more water.

I hope this has helped shed light on keeping your avocado trees healthy and productive. There is a ton of information on this subject. Click here for one one of the many articles I’d recommend.

Questions are always welcome, and I’ll attempt to respond in a timely manner.

— John Schnackenberg is owner of La Palta Orchard Care. Click here for more information, e-mail .(JavaScript must be enabled to view this email address) or call 805.729.6873.

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