Old crop, new crop.  What's up there in the trees?  Are they big enough to sell?  Is there a good set for next year?  These are questions every avocado grower has every year, and often all year long.  What is up there in the trees is confounded by what is called the "Avocado Illusion". 

And boy was I reminded of the issue the other day when harvesting a GEM planting density trial.  You don't see GEMs, you feel them, sense them being somewhere near your hand.  There's a mass that's different from all the leaves near your hand, and you reach for it with your clipper and by golly you got a live one.  But how many have you missed?  You really need to search.

In a Science Magazine Letters to the Editor in Dec 1990, Paul Sandorff commented on a book written by Maurice Hershenson called The Moon Illusion. In the book Hershenson described the illusion of why the moon seemed so much larger when it was on the horizon than when it rose to its zenith on the same night. http://science.sciencemag.org/content/250/4988/1646.1

Sandorff said that this illusion applied to avocados since it was so hard to gauge the size of avocados when they were in the tops of the tree canopy. It is the surrounding environment that puts a context to size according to this theory of illusion.

Hershenson added to this observation in the March 1991 Science letters section with the comment that the leaves surrounding the fruit changes our depth perception and so changes our idea of the fruit size.

A further addendum to the avocado illusion theory is that since the fruit are the same color as the leaves (they are both dark green and the fruit unlike most other fruit continues to photosynthesize), it is hard to actually make out the fruit. You can be looking right at the fruit and not see it, confusing it with a leaf.

This illusion makes for difficult fruit estimation. To compensate for this illusion, I will eye the canopy in quadrants, counting the number of fruit, then arbitrarily doubling that total number. It usually gives a pretty close number to the real number of fruit that are in the tree.

By the way, with all the low down fruit in the skirt and with the wet winter, there were a heck of a lot of snails in the canopy dining on fruit.

Photo:

Can you count the number of fruit in this Hass canopy?