Fruit Drop
I sent the following question to the kind folks at http://www.avocado.org: "I have 2 avocado trees, a Hass (about 4 feet tall) that was planted nearly a year ago and a Bacon (about 5 feet tall, growing nicely) that is in its third year (don't know how long it was at the nursery before). Last year the Bacon had 5 small fruits that fell off the tree when they were still smaller than a pea. This year we had about 10-15 fruits at first, but most of them have since fallen off. I regularly check the moisture content in the top 4-6 inches of soil, and it has typically been moist without sticking in a ball when I compress it. Hollister gets a fair amount of wind. Is this normal for much of the fruit to drop at pea or almond-size? One fruit is nearly the size of a walnut right now, but I wonder if this will drop too? I also visited http://www.crfg.org/pubs/ff/avocado.html, which recommends a number of publications for reading. "
Here is their response:
It sounds like you are having a "fruit drop" problem. There are several factors that can cause fruit drop in avocados. First of all, the avocado sets more fruit than it can carry to maturity so the trees naturally lose some fruit when they reach about bb to marble size. There are at least four other things that can cause fruitlets to drop:
1. Don't give your tree any fertilizer with Nitrogen during bloom season (late Feb - April). This will result in increased vegetative growth at the expense of fruit (some will fall off the tree). After April it is usually considered safe to start applying fertilizer containing Nitrogen.
2. Soak the soil well, and then allow it to dry out somewhat before watering again. Remember that the avocado is a shallow rooted plant with most of the roots in the top foot of soil. People with avocado trees planted in lawn areas typically have a lot of fruit drop because a lawn is watered too often for the avocado.
3. The new avocado thrips will feed on the stem and cause fruit drop. The pest damage and the natural fruit fall occur at the same time, so it is hard to distinguish the two unless inspection is made of the fruit for the thrips. They would be found on the fruit, under the calyx. When fruit gets to about 2 inches in size it is out of harm's way. Next year a spray of sabadilla during fruit set and a second spray about 2 weeks later should control the damage if it is caused by thrips.
4. You can't do much about poor weather conditions during flowering but what happens is that cool conditions will prevent proper pollination. The fruit will grow about 2 inches long and look like small cucumbers - and will often fall from the tree. Sometime these small fruit will stay on the tree and can be harvested and eaten just like regular avocados (they are even marketed as cocktail or finger avocados).
The later sounds most like what you are experiencing. The avocado tree is a native understory tree to the humid and semi-humid tropics. It seems to do best at moderately warm temperatures (60 - 85F) and moderate humidity. It can tolerate temperatures, once established, to around 32 - 28F (0 to -2.2C) with minimal damage. You might try growing a more cold resistant variety outside. Some of the more cold resistant varieties are: Bacon, Mexicola, Zutano, and Fuerte. Keep in mind that avocados are a subtropical plant and while some can tolerate more cold than other, none of them can take truly cold conditions.
Another great source for information on growing avocados is the University of California's Avocado Information site: http://ucavo.ucr.edu/ Good luck and thank you for your interest in Avocado.org
Yellow Leaves
If your leaves are turning yellow, most likely the tree has been over-watered.
Watering Regimen
My avocado trees are still small, so I water them twice a week, about 2 gallons of water each time. As avocado trees have shallow root systems it is only necessary to check the moisture content in the top few inches of soil. If the dirt sticks in a ball when you squeeze it, you probably don't need to water yet. If it crumbles apart, then watering is appropriate.
Covered Crown
It's best to plant your avocado in a mound. Unfortunately, I have mine in a basin. The basin slowly fills with mud and organic debris over time, and the crown of the root system becomes further submerged. This crown, as I understand it, should be right at ground level. The trunk should not be covered by dirt, or else your tree will get very stressed, and this may contribute to browning and drying of the ends of the leaves.