Avocado Tree Biology: How Roots, Pits, and Beneficial Insects Protect the Grove

An avocado grove protects itself in three main ways: shallow feeder roots that need moist, airy, covered soil; a seed that starts the tree while grafting decides the fruit; and predator insects that keep the leaf pests in check. Our junior farmer boiled that down to roots, pits and good bugs, and got it right.

Listening to the youngest members of the family talk about the grove is a good reminder that the basics never change. Here is a closer look at each one.

The role of avocado tree roots

As our junior farmer pointed out, the roots hold the tree up. They do a lot more than that:

• Shallow feeder roots: Of the feeder root length, 80 to 90 percent sits in the top eight to ten inches of soil, according to Avocado Production in California: A Cultural Handbook for Growers, Book Two. That is why we leave the fallen leaves on the ground and add mulch on top.
• Few root hairs: Feeder roots have very few root hairs, so they take up water poorly and the tree needs plentiful moisture across the whole root zone. Too much is just as bad. The roots begin to die from lack of oxygen after 48 hours of flooding, according to Avocado Production in California: A Cultural Handbook for Growers, Book One.
• Anchoring on slopes: Many California groves sit on hillsides, where cold air drains away and water does not stand. Root rot is much worse in poorly drained soils, so the drainage matters as much as the frost protection, and healthy roots hold the soil and the tree in place in strong wind.

Good bugs vs. bad bugs

When our little grower describes the good guy bugs fighting the bad guy bugs, that is integrated pest management in its simplest form.

Persea mite and avocado thrips are the two key pests of California avocados, according to Book Two. Persea mites feed under silky nests on the underside of the leaves, and heavy populations make the tree drop leaves early, which leads to sunburned bark and fruit and fruit drop. Avocado thrips feed on young fruit and scar the skin, which hurts the packinghouse grade but does not affect the fruit inside.

The beneficials that work against them:

• Predatory mites: Neoseiulus californicus and Galendromus helveolus hunt persea mites, and both are sold for release. The handbook is honest that most predators struggle to reach mites inside their webbed nests, so releases help rather than cure.
• Green lacewings: Their larvae feed on both persea mites and thrips, and they are sold for release too.
• Predatory thrips: Franklinothrips orizabensis is the key predator of avocado thrips.
• Parasitic wasps and flies: Tiny Trichogramma wasps lay their eggs inside leafroller eggs, and tachinid flies parasitize the caterpillars. Together they keep leafrollers from stripping branches.

Why can spraying make pest problems worse?

Broad-spectrum sprays kill the predators along with the pests. The handbook notes that caterpillar outbreaks commonly follow malathion sprays, because the spray poisons the parasites and predators that were holding the caterpillars down. Dust does damage too: dusty conditions make parasites and predators less effective, which is one reason growers water dirt roads and drive slowly in summer. Weeds matter as well, because they harbor persea mites.

Why does persea mite hit Hass harder than Fuerte?

Persea mite does the most damage on Hass and Gwen, while Bacon, Fuerte, Lamb Hass and Zutano are much less affected, according to the handbook. Mite numbers are lowest around March, build on the spring flush and peak in July and August. A run of days at 100°F or more with low humidity can crash the population. That makes summer the time to watch Hass trees most closely.

Will a tree grown from a pit give you the same avocado?

No. Every avocado seed is genetically new, so a pit from a Hass does not grow a Hass, and the vast majority of seedlings are inferior to their parent, according to Book One. A seedling may take up to 10 years to bear, and some never do. That is why commercial trees are grafted: wood from a known variety is joined to a rootstock. Grafted trees mostly bear in their third year and reach full production in years eight to ten.

The pit still does its job. It feeds the young seedling until it has roots and leaves of its own, and sprouting one on the windowsill is a great project with kids. Keep it moist but never waterlogged, and treat whatever grows as a houseplant with a surprise ending rather than a fruit tree you can count on.

Raising the next generation of growers

Farming is more than irrigation lines and harvests. It is a running lesson in balance, from the roots to the canopy, and teaching our kids how the grove works is how that knowledge stays in the family.

How the tree protects itself is the backbone of our guide to growing avocado trees. Keeping that root zone covered is the subject of why we leave the leaf duff and add mulch, and the insects we lean on show up in our persea mite release.