Planting an Avocado Tree or Two
© 2006, 2011 R. Frost, San Diego CA.
Practices in planting Avocados have changed dramatically over the decades. Much has been learned in both field studies and biological research. The new commercial practice today is to plant on a mound built over a shallow hole. The mound is buried in mulch with ample water supplied almost daily on top, with frequent applications of a lower phosphorus fertilizer. It is also common to find two or more trees in a hole – one type A and the other type B to boost overall fruit production.
So, you want to grow Avocados at your home? Take advantages of the latest recommendations from Avocado researchers and improve your chances of long-term success.
Tree selection. The most important distinction among Avocado varieties is their bloom type: A and B. One is female blossom in the morning, and male in the afternoon – the other is the opposite. After the sex change, the blossom is often viable for another day. It has been repeatedly observed that groves with both types of trees in close proximity have significantly higher yields. Conversely, some plantings with only one bloom type never produce. Therefore, if your tree will be the only one in the neighborhood, then having an A and B tree in close proximity at home is highly recommended.
Another factor in tree selection is size. A “standard” tree will want to grow 25-40 feet high and most of these just as wide – although the “Reed” variety is well-known for its narrow, upright form. Avocados which have been grafted onto rot-resistant rootstocks have been found to limit their height to the 20-25 foot range. There are also small, “dwarfish” varieties including “Gem”, “Gwen”, “Holiday”, and “Sharwil”. These will grow from 10’ to 15’ high. The “Don Gillogly” variety has been advertised as dwarf, but trials at Durhling’s Nursery in De Luz have proven otherwise.
The taste of the “Hass” Avocado is well known to consumers, especially since distributors prefer the “Hass” variety for uniformity of size and durability in the marketplace. Generally, taste is a function of oil content and texture, the higher % oil being more desirable. The “Hass” Avocado is roughly 72% flesh by weight with 18% oil content. All varieties listed below are over 70% flesh by weight and have oil content of 18% or higher. So, although you may be familiar with the name Hass -- it is not necessarily the first choice of those who taste avocados for a living. Among that group (myself included), the recommendation is often to plant Reed for its rich taste. If you need a pollenizer for it, then Stewart is a great choice. Otherwise if you are fortunate to have room for another tree, I recommend you plant Pinkerton for the nutty overtones in its flavor.
Another Avocado well known to consumers is "Fuerte". This variety was traditionally grown in near-tropical regions as a pollenizer for more pallatable types. About 1 row of Fuertes were (and still are) planted for every 2-3 rows of marketable varieties, and the Fuerte fruits are thrown away. When Avocado farming began in southern California, this practice was unacceptable to growers. A solution to this marketing problem was sought, and thus guacamole was invented for the purpose of selling an otherwise somewhat watery less-than-desirable tasting fruit. Nowadays, we have better choices for pollenizers. However, the marketing scheme has back-fired because it created generations of US consumers who now seek the Fuerte. If this is you, then go to a Farmers Market or the Fallbrook Avocado Festival when the fruits are in season and indulge yourself in some tree-ripened avocado tasting.
Well, we’re not quite done yet. Avocado trees generally blossom in January through March, but there are some significant differences between cultivars. There are 3 main strains of Avocados, with lots of inter-strain hybrids: Mexican, Guatemalan, and West Indian. The latter two flower mid-season – and sometimes all season, while the pure Mexican strains flower early. Obviously it is not much help to have A and B trees that flower in different months.
Finally, Avocado trees can be very susceptible to root rot. If you live in an area with less than perfect soil, then paying the extra money for a cultivar grafted on a good rootstock is well worth the investment. Zutano is the most often recommended rootstock because it adapts to many adverse conditions. The fruit is good quality and thus as a pollenizer it is a reasonable choice. A third use for Zutano is for the hobbiest who wants to graft many different Avocado varieties onto the same tree.
|
Select Avocados for Home Orchards |
||||
|
Tree Size |
Blooms |
Bloom Type |
Variety |
Ripens |
|
small |
mid |
A |
Gem |
summer |
|
A |
Gwen |
summer |
||
|
A |
fall |
|||
|
B |
Sharwil |
spring |
||
|
large |
early |
A (far inland) |
fall |
|
|
B |
Sir Prize |
summer |
||
|
mid |
A |
Ardith |
summer |
|
|
A |
summer |
|||
|
A |
fall |
|||
|
A |
winter |
|||
|
A |
fall |
|||
|
B |
Hellen |
summer |
||
|
B |
Nimlioh |
fall |
||
|
B (coastal influence) |
fall |
|||
|
B |
winter |
|||
Planting. The basic idea is to excavate an 8-foot diameter hole to a depth of 1 foot, and then build in and above this hole a spherical shaped mound that is eventually 1.5 to 2 feet above the original ground level. The trees (type A and B) are planted off-center but near the top of the mound. The entire mound is covered with about 4 inches of orchard mulch.
Dig the hole, keeping all the native soil for later use. Fill the hole with water (quickly, using a high-pressure line) two times, the second time soon after the 1st drains. If the water does not drain away both times within 10-15 minutes each, then there is a drainage problem. Construct one to three “french drains” (pipeless, with ½ to ¾ inch gravel and pumice on the bottom) and test again in 2-3 days.
Remove all cobblestones from the excavated dirt but leave any rocks smaller than a tennis ball. The volume of soil required to refill the hole and build the mound will be
or about
cubic yards.
If your hole and hence your native soil did not pass the “drainage” (percolation) test above, then half of the soil volume should come from an additive with the following properties:
· good drainage
· low in phosphorus
· little or no peat moss
Note that most commercial compost products are high in phosphorus. A good choice is bulk potting soil, which is roughly
· 
washed sand or small grain inorganics
· 3/8″ washed pumice or perlite
· small grain forest
humus or ground coir
If not already present, also mix in 1 bag (1–2 cu.ft.) of humus with worm castings. If your hole did pass the percolation test, then make up the difference with more of your native soil, and use the additive mix above if necessary.
Keep in mind that there are 27 cubic feet in 1 cubic yard. Buying a yard’s worth of soil by the bag is an expensive proposition. Instead, contact a bulk soil supplier or see the “pro” desk at a large nursery or building materials store.
When you have completed filling your excavated hole, place your potted trees (or better: empty pots the same size) where the trees will be planted. A good choice is 1.5 each side of the center, with the trees positioned east and west of each other. Now build your mound around them. When all of the soil is in place, pull out the pots. Sprinkle a small quantity of food at the bottom of the resulting holes, either:
· 1 Tbsp. citrus fertilizer, such as Gro-Power 8-6-8 Citrus & Avocado Food
or
· 1 cup of a low-phosphorus composted manure, such as that from horse or sheep.
Bury the food in a few inches of soil, as a present to growing roots and to bring the soil level of the planted tree up to the soil level of the mound.
Cover the mound with 4 inches of orchard mulch. This will require
of material. Be careful not to encroach on the tree trunk. Important: top mulch is the key component in planting an Avocado. The mulch will provide a home to bacteria which in turn will dramatically reduce the risk of root rot. Check and restock the mulch as necessary on a yearly basis. Soak down the mulch with water to prime it with moisture for subsequent irrigations.
First-year fruits. Remove any fruits that form to the size of a grape or larger in the first year – or that are already present on the potted tree. The amount of energy required to bring a fruit to maturity is about equivalent to 4 feet of branch growth. Allow the tree to go through its fertility cycle, but remove any first year fruits in favor of branch production.
Feeding. Avocados benefit from monthly feedings. They are sensitive to salts, alkalinity in particular, and tolerate only a limited amount of phosphorus. In San Diego county, the ground and municipal water supply are slightly alkaline. Therefore your fertilizer should have a mild acid component that the water will always interact with. Fertilizers that contain humus or humic acids – a group of weak carbonic acids, will do just that.
If you are watering with soaker hose and feeding with a granular, simply pour the fertilizer directly on the soaker hose. Liquid fertilizers that are applied through an in-line feeding system (a fertigator) can be both labor-saving and very effective at treating water.
In terms of N-P-K (Nitrogen, Phosphorus, Potassium) the Avocado needs significant amounts of Nitrogen, limited amounts of Phosphorus, and enough Potassium to ward off disease and decay. In addition, the Avocado requires a host of micronutrients, including magnesium, sulphur, iron, manganese, and zinc. In choosing a fertilizer, it should be highest in Nitrogen content, have equal or less quantities of Potassium, and still less Phosphorus.
The following sources meet these guidelines, and there are undoubtedly more:
· Grow More 28-8-18 Water-Soluble Fertilizer
· Dyna-Gro 9-3-6 Foliage Pro Liquid
· GroPower 8-6-8 Avocado & Citrus Food
· Composted Horse Manure, ~ 0.7-0.3-0.6.
Composted sheep manure (.7-.3-.9) can also be used, but nitrogen supplements are needed to balance the excess potassium.
A small Avocado straight out of a 5-gallon pot needs about ½ pound of nitrogen per year. Older trees, and trees from larger pots, need 1 pound of nitrogen per year. Trees with canopies exceeding 10 feet wide desire 1.5 pounds or more of nitrogen per year. The “N” in N-P-K is % available nitrogen by weight. Using a little proportional arithmetic, the following feeding schedules can be calculated:
|
Monthly Feeding for one Avocado Tree |
|||
|
example product (choose one) |
5-gal. tree |
4'×4' canopy |
15'×15' canopy |
|
3/8 cup |
3/4 cup |
1.5 cups |
|
|
Dyna-Gro 9-3-6 (liquid) |
1.5 oz. |
3 oz. |
4.5 oz. |
|
GroPower 8-6-8 |
1 cup |
2 cups |
3 cups |
|
Composted Horse Manure |
1 gal. |
|
|
I personally recommend the Grow More 28-8-18 for the high quality of nutrients, ease of use, and low cost per net weight of nutrients.
Watering. Install your irrigation with either flexible irrigation hose or an array of sprinkler heads.
For flexible hose (e.g., Fiskar 5/8″ soaker), lay it out in a spiral starting about 1 foot from the base of the trees and expanding outward at 1 foot intervals. Optionally, insert 2 to 4 gallon/hour pressure-compensating drippers in the hose at 16 inch intervals. The hose is essentially an Archimedes' Spiral (polar equation r = aθ) with r = 4 feet after 4 complete rotations, or θ = 8π. The length of this spiral will be
feet
optionally using about
38 pressure-compensating drippers.
For sprinkler heads, install two 270° ~3 gal/min heads – one at the east and the other at the west side of the mound, so that the “notch” points at the tree trunks; i.e., no spray will hit the trunks. Also install two 180° heads – one at the north side and the other on the south side of the mound, again pointing away from the trees. Adjust the water pressure so that the spray area matches the tree canopy.
The actual amount of water to use depends on (a) the climate you live in and (b) the tree size – or really the ground area circumscribed by the tree canopy, and (c) several other factors discussed in the references listed at the end of this article.
A key factor in irrigation analysis is the local ETo (evapotransporation) rate. As shown in the map below, there are 6 major ETo zones in San Diego county:
1. Heavy fog belts
4. Coastal hills and mesas
6. Intra-coastal ridges (e.g., Scripps Ranch)
9. Interior valleys with semi-arid transitions
16. Mountainous regions
18. Eastern low-desert areas
ETo Zone Map for San Diego County
The CIMIS (California Irrigation Management Information System) irrigation model for Avocados is:
where:
G, the gallons of water is computed in months since ETo and Kc
are also monthly figures. The tree canopy size for a new tree out of a 5
or 12 gallon pot can be considered 
, or 16 sq.ft.
A 5-7 year old tree with a 20 foot canopy will have C = 400 sq.ft. The irrigation efficiency 
can be considered
equal to 1 if you are using emitters with a guaranteed flow rate.
Putting this altogether, one can obtain estimates for irrigating in your specific zone on a monthly basis. The charts below show estimated watering requirements for two tree sizes in Zones 1, 4, 6, and 9. However, zones 4 and 6 are so close in calculated value, they have been averaged into a single column.
Although these tables show monthly irrigation amounts, water is usually applied 2 to 3 times per week – the exception being during wet weather. The CIMIS formulation above does not account for precipitation. A rule of thumb is that irrigation should be delayed about one week after each storm of ¼″ or more of rain.
Gallons / mo. for a young tree, 4'×4' canopy
Gallons / mo. for mature tree, 20'×20' canopy
References.
Arpaia, Mary Lu; et al. "Avocado Varieties." From Avocado Information, University of California Agriculture & Natural Resources. http://www.ucavo.ucr.edu/
Bier, O. UC Riverside Botany & Plant Sciences Department. Personal interview. Mar. 11 2006.
Del Hotal, T. “Tempting Subtropical Fruits For Your Garden Of Eden.” Presentation. Master Gardener Association of San Diego County. USD Campus, San Diego, CA. Mar. 11 2006.
Eames-Sheavly, Marcia; et al. “A Guide to the Nutrient Value of Organic Materials” Cornell Gardening Resources, Ecogardening Fact Sheet #8, Spring 1993. http://www.gardening.cornell.edu
Frink, Julie. “The Avocado.” Presentation. California Rare Fruit Growers, San Diego Chapter. Balboa Park, San Diego, CA. Oct. 26 2006.
Goldhamer, D.A. and R.L. Snyder (eds) “Irrigation Scheduling – A Guide for Efficient On-farm Water Management”. University of California Division of Agricultural and Natural Resources. Publication No. 21454. 1989.
May, Sharon. Agri Service, Inc., Vista CA. Personal interview. Nov. 27 2006.
McLaurin, W., Reeves, W. “How to Convert an Inorganic Fertilizer Recommendation
To an Organic One” University of Georgia Cooperative Extension Circular 853, Nov. 2000.
http://pubs.caes.uga.edu/caespubs/pubs/PDF/C853.pdf
Snyder, R.L., et al. “ETo Zones Map” California Irrigation Management Information System, 1999. http://www.cimis.water.ca.gov/cimis/info.jsp
Spellman, T. “Is Your Avocado a Type A Personality?” Garden Compass May–June 2003.
Weisstein, Eric W. "Archimedes' Spiral." From MathWorld–A Wolfram Web Resource. http://mathworld.wolfram.com/ArchimedesSpiral.html
Weisstein, Eric W. "Spherical Cap." From MathWorld–A Wolfram Web Resource. http://mathworld.wolfram.com/SphericalCap.html
Witney, G., Martin, G. “Taking The California Avocado Breeding Program Into The Next Century”. In Proc. The World Avocado Congress III, 1995 114–118.