Avocado trees in cool climates

Alhough avocado trees can survive in the cool temperate climate of the southern PNW coast, they do not fruit there naturally. Without heat during the January to March flowering season, and cooler temperatures during winter to help blooms set, these trees do not develop blossoms that yield fruit. Only with a greenhouse and thoughtful care will these trees thrive and offer avocados to Oregonian guacamole supplicants.

Even when avocado trees blossom abundantly, a lack of pollination will lead to zero fruit. This is not a problem in southern California where avocado pollen floats in the breeze. In a PNW greenhouse, however, chance of pollination increases significantly if the building holds both A and B tree types. These two types release pollen at different times of the day and tend to pollenate each other more successfully than they can pollenate themselves.

Avocado trees require well-draining soil (loamy, rich, and shallow) and prefer full sun. An avocado tree planted in-ground will benefit from well-draining soil around its roots. (See this guide for tips. Health of these trees may also improve when their fallen leaves are left in place; these appears to retard fungal growth.

The smaller species with good flavor are the Holiday (a dwarf), the Stuart (aka Stewart) Mexicola, and the Lamb Hass. The much-marketed Don Gillogly type has not fruited well (or at all) for most growers and should be avoided unless better reports emerge. The LittleCado does not appear to yield optimally flavorful fruit, but can apparently produce fruit in a small space if given sufficient heat and light.

Some avocado trees can be pruned and possibly trained so that they will not exceed 20 feet, but pruning may diminish fruiting. The Holiday variety can naturally be as small as 10 - 12' when full grown, but other reports have suggested taller heights for this variety (15' or even 20'). Trees grafted onto rot-resistant stock typically do not grow taller than 20 - 25'. Pruning should be done only at specific phases of the tree's growing cycle and with careful techniques because pruning at the wrong time or with the wrong method can destroy/decrease fruiting. These trees sometimes grow quickly: the popular Hass species can grow to 25 feet in 15 years.

Most avocados sold by nurseries are grafted onto a root stock optimized for local conditions. The Zutano root stock is popular in California and might suit the PNW, but the nursery should be consulted for advice. This root stock likely diminishes maximum height, thus decreasing need for soaring greenhouse construction.

At present, the best chance for successfully fruiting avocados in the southern Oregon coastal area appears to be a combination of greenhouse, heat, lighting and well-chosen tree types and root grafts. My recommendation:

Purchase 2 or 3 medium-sized trees grafted onto the root the nursery suggests (likely the Zutano). At least one of these should be the Holiday, the others could be the (slightly preferred and possibly compact) Stuart Mexicola and/or the (potentially densely yielding, possibly compact) Lamb Hass. Ask the nursery for recommendations. Best option for second tree is a multi-grafted combination of Stuart Mexicola and Lamb Hass if one is available--it doesn't hurt to ask. I would nurture these trees staked in 1/2 whiskey barrels or large terra cotta pots at first, keeping them in a greenhouse environment that prevented freezing, and ensured heat and light during the important budding and fruiting season.

Avocados do not fruit well in containers, so as soon as appropriate I would transplant the avocado into a hole dug and prepped (see above link) to maximize soil quality. I would follow Pat Welsh's transplanting instructions and destroy the container with the tree already placed into the hole to avoid damaging the roots as much as possible. The spacing of trees from each other is a controversial topic, but could be as near as 4' or as far as 10'. Depending on how far the trees have spread, I would offer more or less space to minimize crowding while maximizing cross-pollination. The wider the trees are located, of course, the bigger the greenhouse enclosure must be. This is a big consideration. Nonetheless, the overhead involved in growing avocado trees at all might justify the cost and effort of a slightly wider greenhouse if it means far greater fruit yield.

Avocado trees are apparently nutrient hungry and need water, but cannot tolerate sogginess and can also get burned easily by the sun. Attention will need to be paid in the early years to be certain that healthy roots and leaves are nurtured. This could lead to long-term hardiness and might allow gradual weaning off the greenhouse structure during parts of the year.

To accomplish this lofty feat, I suggest an adjustable/removable greenhouse design for the avocado zone. Not only would it be impossible to guess how tall and wide the trees might become (even seasoned gardeners on forums do not agree), creating a structure for the maximum likely height and width would require a hefty investment (and heating lots of extra space) for years before avocados arrived in abundance. This doesn't make logical sense when an adjustable arrangement could work just as well.