Answers to Questions
by Dr. Mary Lu Arpaia and Dr. Ben Faber
Questions on avocado culture
The avocado is a shallow rooted tree (most of the feeder roots are in the top 6" of soil) which needs good aeration. They do well if mulched with a coarse yard mulch. Current recommendation is to put approx 1/3 cu yd per tree when planting. When applying the mulch, be sure to stay about 6-8 inches away from the trunk of the tree. They like the soil pH around 6 - 6.5. If you can, plant your tree in a spot protected from wind and frost. Also, avocado trees typically do not do well planted in lawns so try to plant your tree in a non-lawn area.
- When should I plant my avocado tree?
Avocado trees like warm ground. Ideally, they should go into the ground from March through June. If they go in during the summer there is always the risk of sun damage because the trees can't take up water very well when young.
- How big a hole should I dig?
As deep as the current root ball and just as wide as the width plus a little extra so you can get your hands into the hole to plant it. Don't drop the tree into the hole, the roots don't like that, ease it into the hole. The avocado root system is very sensitive and great care should be taken not to disturb the root system when transplanting. If the tree is root bound, however, loosen up the soil around the edges and clip the roots that are going in circles.
- Should I put some gravel, crushed rock or planting mix at the bottom of the hole?
No. Do not put gravel or anything else like planting media in the hole. The sooner the roots get out into the bulk soil, the better the tree will do. Planting mix creates a textural difference between the root ball and the bulk soil and causes water movement problems. Remember, there are 5 million acres of tree crops in California planted without planting mix.
- I have a heavy clay soil. Should I elevate the tree in a mound for better drainage?
Yes, good idea. Make the mound 1 to 2 feet high and 3 to 5 feet around. Put down 20 pounds of gypsum spread around the base of the tree and mulch the area with 6 inches of woody mulch keeping the material about 6-8 inches away from the tree trunk.
- What do you mean by a "coarse yard mulch" and where can I get some?
Redwood bark will work and maybe cocoa bean husks and shrededed tree bark. Need something that is woody and about 2 inches in diameter. Coarse yard mulch is available at some garden supply centers. Be sure it is COARSE, not fine, yard mulch - and disease-free to prevent introducing diseases to your tree (like root rot). Another source of coarse mulch would be a tree trimming operation, like Asplundh or Davy. They usually have material that has been pruned from the tops of trees and doesn't contain any diseased roots. Just go through the yellow pages looking for tree services.
- How much sun is best for my tree? Full sun, morning, afternoon?
Full sun is best.
As far as fertilizing, the recommendation for young avocado trees is 1/2 to 1 pound of actual Nitrogen per tree per year. You can spread it out over several applications as long as it totals 1/2 to 1 pound of Nitrogen. The other important nutrient for avocado trees is Zinc. A general use home fertilizer that is used for houseplants normally should work. You may once a year wish to feed in some zinc if the fertilizer you are using does not have zinc. The major nutrients that the avocado tree needs are NPK and Zn.
- I use an organic avocado food labeled 7-4-2. I do not see zinc listed. What does NPK mean?
NPK means Nitrogen, Phosphorus, Potassium. So in your 7-4-2 fertilizer you have roughly 7% Nitrogen, 4% Phosphorus and 2% Potassium. There are organic fertilizers that contain Zinc (Zn) but you'll have to look for them, it need not be labeled "avocado food." I am surprised that a fertilizer labeled "avocado food" would not contain Zinc.
When watering the tree, it is best to soak the soil well, then allow it to dry out somewhat before watering again. Of course, like most plants, you don't want the tree to get too dry! The rule of thumb for mature trees is about 20 gallons of water a day during the irrigation season. Seedling will require quite a bit less than that, of course. At planting the trees can hold about 2 gallons of water in their rooted volume. Depending on the weather, they might use 1 gallon of water a day along the coast. Typically the trees need to be watered 2-3 times a week. As the roots reach out into the bulk soil, more water can be applied and frequency of watering diminishes to about 1 time per week by the end of a year. Check the soil before watering, to make sure it has dried somewhat. If the soil from around the roots can still hold the impression of the hand when squeezed, it still has enough water. To help you calculate the amount of water for a small tree, you can look at the irrigation calculator at: http://www.avoinfo.com I believe it is located in the grower section.
Avocados are not easy to graft so my first suggestion would be to take the trees out and purchase the varietys you want from a local nursery. If, however, you want to graft onto your existing trees, I would suggest you hire someone who does this on a regular basis. If you live in an area where avocados are being grown you should be able to locate someone with this skill by contacting a local avocado grower. Another option is to contact a nurseryman and see if they know anyone who works with avocados. A local arboretum might also know of someone. In California you might try contacting the California Rare Fruit Growers. You might be able to reach Rare Fruit Growers through their state web site at: http://www.crfg.org/
If you are determined to try grafting yourself, check the following site to see if they have a pamphlet that you can purchase about grafting avocados. http://danrcs.ucdavis.edu
For grafting, Dr. Mary Lu Arpaia, Extension Subtropical Horticulturist, suggests the following manual: Propagating Avocados: Principles and Techniques of nursery and field grafting. Publication #21461.
Growing from seed
It is possile to grow an avocado from seed, just don't let it dry out. Be aware that the seed is the result of cross-pollenation so the resulting tree will be different from the tree the fruit came from. For example, if you plant the seed from a Hass avocado, the resulting tree will be a cross between a Hass and something else... it will NOT be a Hass! Also, keep in mind that avocados planted from seed take anywhere from 5 to 13 years+ before they flower and produce fruit. When I start an avocado from seed I usually take it right from the fruit, I cut about 1/4" off the tip of the seed with a sharp knife, and place the seed in a pot with potting soil with just the flat/cut top of the seed showing above the soil. Keep it moist and wait... (time to germinate varies).
Pollination & Fruit set
The avocado flowering patterns fall into two groups: "A" type and "B" type flowers. A type flowers open female in the morning and male in the afternoon, B type are male in the morning and female in the afternoon. The question many people have (and which has not been fully answered yet) is, should I plant a "B" type avocado with an "A" type avocado to help with good pollinization?
It is widely accepted that fruit production can be helped with the presence of another avocado variety. Temperature during bloom can also influence fruit set. Optimum fruit set occurs at temperatures between about 65 - 75 F. Cooler or warmer temperatures are less ideal. Under some conditions you may get a fruit from a flower that did not pollinate properly. These small, elongated fruit will often fall from the tree on their own, but if they "hang on" you can pick them and eat them. These fruit are called "cukes" but are sometimes marketed in stores as "Cocktail" or "Finger" avocados
The avocado tree typically can produce up to about one million flowers but will only typically set about 100 to 200 fruit per tree.....or in other words, 1 fruit in 10,000.... Sometimes they will set fruit but then drop them when they are pea to walnut size...again this is typical. What can be done to minimize fruit drop of good "fertilized" fruit. Well, avoid stressing the tree, that is don't under or overwater the tree. There has been research in Israel which suggests that fruit retention is also facilitated when there are other avocado varieties present to provide cross-pollination and that these crossed fruit have a higher tendency to stay on the tree. There is also some indication that overfertilizing with Nitrogen during the early fruit stages can also somewhat influence this practice, although this data is not overwhelmingly convincing. I would suggest not to fertilize with nitrogen from about April through mid-June, or apply very low amounts during this time.
- Recently my tree has been dropping small size avocados. It appears as if the stems are being damaged somehow. What's causing this?
When fruit drops from natural causes it will normally form an abcission line on the stem. However, the new avocado thrips will also 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 an inspection is made of the fruit for the thrips. They would be found on the fruit, under the callyx. By the time this information gets to the person, the damage will have already been done. 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.
- What are the climate requirement of an avocado tree?
The avocado tree is a native understory tree to the humid and semihumid 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 with minimal damage.
- How many different kinds of avocados in the world?
Keep in mind that each seed planted results in an avocado tree very different from the parent tree. For example, you plant a seed from a Hass avocado... the seed is the result of cross-pollination and the resulting tree and fruit will be very different from its parents. Since this is the process by which new varieties are discovered, there are literally thousands of named varieties of avocados (and untold unnamed ones in the wild). In fact, there is a lady who has volunteered to go back through the literature and compile a list of named varieties for us, so far she has over 1000 varieties in her database! However, there are really only a small number of commercial varieties being grown for sale today, the main one being Hass.
- How can I tell when my avocados are ripe?
Avocados do not "ripen" on the tree, that is, they do not get soft while on the tree. Once you pick an avocado, it takes about 7 to 10 days for it to soften when left at room temperature. You can speed the process up slightly by placing the avocado in a bag with some other ripe fruit (like an apple) or slow the process down by keeping the fruit in the refrigerator.
As far as knowing when it is ready to be picked, it is hard to tell from the outside when an avocado is mature. What the industry does is called a "dry weight" test which gives you an indirect measure of the oil content of the fruit. If the oil content is too low, the fruit is not ripe yet and will shrivel or stay rubbery insted of getting soft. I suggest you pick a couple of fruit and try to ripen them. If the fruit shrivel up or seem rubbery insted of soft, they are not mature yet. Keep picking fruit every few weeks. Note on the calendar when they soften insted of turning rubbery. Also, note the taste of the fruit. The oil content of the fruit usually increases through the season and there will be a certain point when it tastes "just right." That date will usually vary somewhat due to climate conditions... and some years will be better than others. Some varieties can also reach a point where they have too much oil and some will turn rancid (although many types fall from the tree before reaching that point).
The Hass avocado typically ripens in February and is good through June or July. These dates depend a lot on where you live and climate conditions. Some years you can pick larger fruit as early as December and they will ripen up. Up in Ventura County, fruit can remain on the tree and still be good into August and September.
Varieties that ripen other times of the year (dates based on our plot in Irvine, Ca):
Reed: June - Sept
Pinkerton: Nov - Jan
Fuerte: Dec - March
Lamb-Hass: May - Aug
- I just planted an avocado tree. How long before I start getting fruit?
This depends on several factors. First of all, are you purchasing a tree from a nursery or growing a tree from seed? If purchasing a tree, you can probably expect to see your first fruit 3-4 years after planting the tree. If growing from seed, it can take anywhere from 5 to 13 years before the tree is mature enough to set fruit. When the tree does flower, expect a lot of flowers to fall from the tree without setting fruit. This is natural.
- How many fruit will a mature tree produce in one year?
It is possible for an avocado tree to produce 200 to 300 fruit per tree once it is about 5 to 7 years of age. The avocado however, alternate bears. This means that the tree may produce a large crop one year, and then will produce a small crop the following year. There are lots of variables which will influence this.
- How many years will a normal avocado tree produce fruit?
As far as I know, an avocado tree will continue to grow and produce fruit until something kills the tree. The origional Hass tree (1926) is still alive and producing fruit. There are some wild trees in Mexico that are over 400 years old that are still producing.
- I've just planted an avocado grove. How many years before it becomes profitable?
Typically breakeven is about year 5 but it all depends on cost of water and land, variety, and price paid for the fruit at a conventional yield. You can check out the economics section on the following web site. UCCE Ventura: http://ucceventura.xlrn.ucsb.edu/
- What do the frequent holes and black spots under an avocado's skin mean? Are those bruises and ok to eat? Or are they rotten spots that must be cut out?
You pose an interesting question which is difficult to answer without having a picture of exactly what you mean. The avocado fruit can have black spots and small air pockets under the skin due to mechanical damage during handling...these can be due to the fruit being impacted or compressed when partially ripe. The fruit can also develop brown semi-hard spots under the skin which can also be due to a decay organism. The organism is similar to one which may attack bananas, mangos and a range of other fruits. To my knowledge however it does not produce any deadly toxins so that if you excise the affected area of the fruit it should be safe to consume.
- What can I do about a sooty mold problem?
If it is sooty mold, a fungus that is superficial and is growing on insect exudate from mealy bug or apids, the key is to control the insect. The sooty mold can be just washed off. scales and aphids are controlled by controlling ants. For more information and photographs, visit the IPM web page at: http://www.ipm.ucdavis.edu/PMG/selectnewpest.avocado.html
- What is causing the brown spots on my leaves?
The brownish spots on the leaves is probably a symptom of the persea mite. This little creature showed up in California several years ago and caused a lot of problems including complete defoliation of some trees. The pest has been less and less a problem, probably because some natural predators have started controlling it. Still, if the leaves get a lot of spots on them and start falling off, you might want to try a soap spray or mite spray on the leaves.
- My leaves turned brown and fell off when I moved my tree indoors for the winter.
There's a couple of things that confront a tree going from outside in. One is to insure adequate light intensity. Make sure it is in a bright environment. It shouldn't be in a direct southern exposure, because then it can burn, but as much light as possible without facing the burning rays of the south. Secondly it doesn't like the high/dry heat of a house. Don't exceed 80 degrees and make sure the container doesn't dry out. Occasionally spraying the leaves with water will help.
- My avocado tree has gray/black spots (looks like mold) on the bottom side of the leaves along the veins, and the leaves have started to drop.
This is a case of persea mite. The mites have left and the leaf is about to fall. Hose the underside of the leaves with a good strong stream of water or try a soap spray or mite spray on the remaining leaves.
- Our avocados get "cuke" sized and then form black, mushy bottoms before falling off the tree. What causes this?
Unfortunately, this sounds like avocado bud mite. It has been a problem in So. Ca. for the last 3 years and is likely a result of the odd weather we've been having. There are currently no proven control measures for the problem. It is caused by a miniscule mite that feeds on the forming bud, causing the distortion. A narrow range 415 oil sprayed pre-bloom may help, but thorough coverage is required. If your tree is too large, it may be possible to spray just the lower portion of the tree in order to get some fruit.
- What's the creamy-white foamy looking stuff that grows out of the bark where there are cuts, or small branches have died and dropped off?
It is just sap coming from a wound. It dries that sugary white, fluffy stuff.
- When is the best time to prune avocado trees?
Avocados can be pruned any time of the year, but there tends to be less vigorous regrowth if it is done after cold weather in the winter, sometime around February. Click here to view a video on Pruning Small Avocado Trees by Dr. Piet Stassen.
- How large will my avocado tree get?
Growth is reflected in rootstock, variety, soil depth and texture, windiness, irrigation and pruning. Columnar types (like Reed and Bacon) can ultimately get to be the same size as umbrella types (like Hass), but will take up less room. Both types can go to 35 feet in 30 years. Pruning can keep the trees to a manageable size, under 15 feet, but pruning must be done on a regualr basis, such as in peach.
- I want to grow avocados in a greenhouse. What variety would you suggest?
First of all, be aware that you will have to prune your tree to keep it small enough for your greenhouse. I have seen a tree in Chile that was at least 60 feet tall. But don't worry - I think for your purposes you can keep them pruned to around 12 - 15 feet. You will probably need to prune the tree several times a year but the major topping should be done in the winter time.
Now, as to variety or kind to grow - there are 100's of avocado varieties for you to consider. Given the fact that you want to grow this in a greenhouse you might want to go with a less vigorous variety. The 'Gwen' avocado is a green skin variety that produces a smaller tree. I have seen 12 year old trees that are about 15' tall without pruning. The tree is semi-compact with an average width of approx 10 feet. The fruit in California matures around March and will hang on the tree to approx May/June. Another variety which produces summer fruit is the 'Reed' which is also a green-skin variety. This is a vary upright colummar tree with a smaller average width. I have seen these trees successfully maintained at below 15 feet. The fruit of this variety in my opinion is excellent, especially in late summer (approx 16 - 18 months after flowering). You could also consider the 'Pinkerton' (green skin, early season fruit) which tends more to be a spreading shorter tree. The 'Hass' (the variety that turns black when ripe) is a moderately vigorous spreading tree. A new variety is the 'Lamb Hass' which is also an upright tree. It produces fruit that also turns black when ripe. The fruit matures late spring through the summer.
- What areas of California are most hospitable to avocados?
Most areas of Southern California are suitable for avocados except for the mountains and high deserts wher it gets too cold and too dry for fruit set. Outside of Southern California, it depends on the climate. Cold is most often the problem one faces in other parts of the state. Still, I know there are home growers with avocado trees in and around San Francisco. There is also an area along the foothills of the Sierra Nevada Mountains where avocados are being grown in Fresno and Tulare Counties. Growing areas in Southern California include:
- San Diego County
- Orange County
- Los Angeles County
- Riverside County
- Western San Bernardino County
- Ventura County
- Santa Barbara County
- some parts of San Luis Obispo and Santa Cruz Counties
- I want to know about an avocado known as the "cocktail" or "finger" avocado.
The small, pickle shaped fruit are called "cukes" (as in cucumbers) by many in the avocado industry and result from a flower that did not pollinate properly. These fruit will often fall from the tree on their own, but if they "hang on" you can pick them and eat them. Some varieties are more prone to producing "cukes" than others and occasionally avocado growers will harvest them and market them. When sold, they are often labeled "Cocktail" or "Finger" avocados.
Dr. Mary Lu Arpaia. Extension Subtropical Horticulturist, Kearney Agriculture Center, Parlier, Ca.
Dr. Ben Faber: Farm Advisor. Soils and water, avocados and subtropicals, Ventura County, Ca.
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