Plant Profile- Avocado

This is my first plant profile on a tree crop, despite these being one of my major priorities on the farm. It is an odd first choice since it is currently a marginal species for me. I group my species into established, marginal and experimental. Established species have proven themselves to be locally adapted, reliable and productive and my main focus with them is how to integrate them into wider systems and breed better varieties. Marginal species have shown some promise but haven’t quite jumped all the hurdles yet. Experimental species are new introductions with too little information to make any serious judgements. Only a handful are solidly established, while many are lingering in marginal. I try to limit new experimental species I try each year since they are usually dead ends and take energy away from other species (plus they add some excitement so no point scratching them all off my maybe list at the same time).

My first experiences with avocado were not very encouraging. Back when I had more money than time I did the recommended thing and bought a variety of grafted trees at considerable expense. These required large holes for planting and regular follow up watering during establishment. I refused to go as far as building elaborate earthworks mounds for them as any potential crop would never repay the fossil fuel consumption (conveniently ignoring the trucks driving all those grafted trees over the countryside). Despite taking reasonable care every single one of these expensive trees died within their first year. Digging a hole in our clay soil, then sliding a root ball grown in decomposed pine bark medium, is basically creating a private swimming pool for your plants. If wet weather arrives before the local clay has infiltrated the compost it fills with water, becomes anoxic and the tree dies shortly after. The bigger the tree the more likely this is to happen.

So I took a break from buying more avocado trees and decided to sit back a while and observe. I remembered the towering avocado trees that grew out of the compost heap at my childhood home on volcanic red soil. This cluster of trees produced large amounts of good quality fruit only slightly smaller than those in the shops. I also noticed our neighbour had a 5 m tall self sown avocado tree growing mere meters from where my grafted trees died. It suffered a set back eventually and same experts who told me to buy grafted trees said it was from the taproot hitting the clay subsoil.

I wondered a lot about how those experts knew that was the case. Did anyone ever dig up a dead tree carefully enough to see where its roots had gone (though you would need to do this with trees before they died as well to compare). One thing my training as a research scientist taught me was to recognise the difference between a simple story to explain the world versus actually measuring what you claim is true. That is to say I wasn’t completely convinced that avocado were worth abandoning just yet.

At the same time I wasn’t going to waste more money on grafts. Instead I did a little research and learned how much avocado hate root disturbance. All plants do but we usually ignore the impact, easy to do when so few plants we experience grow from seed direct where they are planted. Even in the vegetable garden people transplant carrot seedlings, often oblivious to how doomed their roots will be (don’t get me started on the nurseries that sell carrot seedlings). There is one group of plants we see all the time that don’t have this handicap- weeds! People often marvel about how unkillable weeds are despite difficult conditions but don’t get as far as wondering what their tricks are other than being evil. The reason we overlook damage to root system structure from transplantation is because it is underground and therefore invisible, plus we almost never see side by side comparison of transplanted versus direct sowed plants of the same species. An easier analogy to grasp is how you can cut the top off a tree without killing it, but the regrowth will never quite function as well again.

The second time around I decided to simply plant avocado seeds direct where I wanted them to grow. They were barely covered with soil to discourage rats but allow the large seed to breath. With my highly diverse soil I decided to plant them in multiple locations. Some places had logical advantages, like the mounded up walls of dams (good drainage?) but I decided to let the plants tell me where they really wanted to grow. I was buying seconds avocado from the local farmers market anyway so the seeds cost nothing more, and planting them only took about 5 minutes each. Seed shrivel if left dry for more than a few days. Burying in a pot of barely damp soil helped build up a supply of seed before venturing out to sow.

I sowed two or three seed in each location with a view to thinning excess trees once they showed signs of fruiting. Those that fruited first with at least average quality fruit would win. I also planted the clusters closer together than mature trees would stand with an aim for further eventual thinning, much like people direct seed vegetables then thin but on a bigger scale of time and space. I even interplanted other species like peach palms (Butia capitata) so if the avocado failed they could use the space instead, saving me starting from scratch years later. I figured if they couldn’t establish this way in a half dozen spots then they probably wouldn’t be productive later anyway, so no big loss and I could give up on the species for good.

Without watering or weeding the seeds germinated. And they grew bigger year after year. The biggest trees are now 4-5 m tall after about as many years. The ones on the dam wall are smaller at about 2 m. Maybe the subsoil brought up from dam construction doesn’t suit them. Or maybe that is just an easy story to tell myself to stop asking questions I can’t answer. Either way it doesn’t matter because my strategy allowed me to find the best places by asking the plants themselves. With rotational grazing of the cows in paddocks uphill a soak appeared close to what are now the biggest avocados. I was pretty convinced this would kill the trees since everyone knows “avocados hate wet feet”. That may prove to be true one day, or maybe not. Again the trees will tell me when they are ready. And if one tree dies then another one elsewhere might be fine. And even if they all die I am only out of pocket the price of the fruit (still delicious anyway) and five to ten years of doing nothing with them and getting on with my life. If all goes well and the trees flower and fruit in a few years then they will move from the marginal to the established category. In some subsequent years almost all have germinated, other years almost none. I could try and figure out why (rain? temperature?) but who cares since it isn’t like I can control those anyway. Just replant again next year.

Avocados have a fascinating history that deserves reflection. They belong to a group of new world trees that were distributed by giant ground sloths, a spectacular species that was rapidly hunted to extinction by humans upon their arrival in the Americas. Humans have taken over the role of the ground sloth and are now responsible for the survival of the tree. In some ways we have done a better job than the ground sloth ever did, spreading the species across the entire planet. A lasting symbiosis depends on forming a true mutually beneficial relationship, something achieved by those Amerindian people as they transitioned from being hunter-gatherers to agriculturalists. The shift to industrial agriculture has produced a more twisted relationship, with humans now forcing the trees into unnatural shapes to make rapid harvesting easier, and packing trees into high densities to increase fertiliser and machinery efficiency. This artificial system is what produces those grafted trees in their thousands, selected for rapid uniform production under conditions made perfect by the consumption of fossil fuels. The varieties are selected for size and shipping quality, and uniformity is prized to maximise efficiency. As a result of this the choices for a zero input farmer are more limited with surplus grafted trees pushed into the home garden market. Even growing from seed I am limited to major commercial varieties, though Hass is reputed to produce the best seedlings. These will at least be outcrossed to a range of other varieties present in the orchard (often only grown for strong pollination traits) but the enormous diversity of the species has been left behind in Central America. As an amateur grower in Australia I cannot access this diversity, meaning that even if my trees start producing I will be lacking genetic diversity and potential for local adaptation. If the trees continue to be grown for generations they will eventually accumulate useful mutations and become locally adapted but only if people are brave enough to grow them from seed. What have you got to lose from trying?

A trio of seeds going in yesterday. Plant and forget (until harvest time in 5-15 years).

A young avocado seedling from last year, poking through the unkempt weeds

Medium sized trees on the dam wall. A few dead on the left, a few alive on the right. Why? Why not?

My largest trees growing down hill from a seasonal soak.