FERTILIZING AND PRUNING OLIVE TREES
Olive trees do not require overly fertile soils. They do, however, require the basic levels of nutrients and micro-nutrients for healthy growth. The University of California has published two books, The Olive Production Manual and The Organic Olive Production Manual. The Olive Production Manual and The Organic Olive Production Manual have informative chapters on olive tree fertilization. These resources will be the best in-depth guide to olive tree fertilization in California.
FERTILIZING LANDSCAPE TREES
For those who maintain a few olive trees in the garden or in pots, a general fertilizer applied annually or semi-annually is often sufficient. At McEvoy Ranch, we use an organic fertilizer in pellet or crumble form that releases its nutrition slowly, applying once in the spring and once mid to late summer. The N-P-K values should be about equal and not exceed 10%. The label of any commercial fertilizer will have the best recommendation for the appropriate rate.
For large-scale growers, it is better to calibrate your fertilizer applications with soil and leaf tissue nutrient analyses. These tests allow you to tailor your fertilization program exactly to your orchard’s needs, not wasting your time or money on unneeded nutrient application.
Soil tests can be taken any time of year and should represent a fairly congruous growing area. Orchards with more than one type of soil should have as many samples submitted so that each sample reflects the distinct soils represented in the grove. Peaceful Valley Farm Supply has a good package of soil sampling instructions and test results interpretation for soil sampling beginners. Opt for the macro and micronutrient analysis as some of the micronutrients are essential for vigorous growth and solid fruit set.
Tissue samples are a very useful indicator for gauging trees’ health. Tissue samples can be taken any time of year, though UC Davis recommends late July through early August because the critical levels of nutrients have been established for that time period. It is important that the time period of sample-taking remain consistent from year to year to build your own comparative data. It is also useful to take samples from groups of healthy trees and unhealthy trees (if you have any!) simultaneously for comparison; comparative samples can be taken any time of year.
For assistance interpreting your soil or tissue sample results or for more information about our consultation services, visit our Consultations page.
Pruning methods vary according to the desired function of the tree. A large-scale orchard must be pruned for efficiency of harvest, while the aesthetic of a garden centerpiece is paramount. The best resource and most extensive and informative book on olive tree pruning is Olive Tree Pruning and Training Systems Manual, by Riccardo Gucci and Claudio Cantini. The University of California’s The Olive Production Manual also has a good chapter on olive tree pruning.
There are a few principles to keep in mind no matter your end goal when pruning. Olive trees will tend to grow a dense canopy without annual or semi-annual pruning. The branches in the middle of the canopy often defoliate due to overcrowding and must be removed regularly for the tree’s health.
Olive trees bear their fruit on year-old wood. Therefore, it is generally preferable to thin branches from the base rather than to tip them, to protect next season’s crop. If you are unsure where your fruit will be, you can wait to prune your trees until the summer when the fruit is visible.
Most pruning takes place in the winter when the trees are resting. Olive trees are evergreen (they do not go dormant in the winter), but their growth slows in the cold months. Most growers opt to prune when the trees are resting and when there is less work to do elsewhere in the orchard, but an olive tree can be pruned any time of year (particularly if fruit production is not the objective). An olive tree that has been pruned in the winter often requires a summer pass to remove suckers and watersprouts. Suckers and watersprouts are much easier to remove when they are still young and tender before they lignify.
At McEvoy Ranch, we train our nursery trees for an open vase formation. The mature open vase-shaped tree has a trunk that branches out at 24–36” into three to five major scaffold branches. The middle of the tree is kept open (the vase part) to allow for ample light penetration and air circulation. An open canopy reduces disease pressure and ripens fruit more evenly.
In a typical tree that starts with good form, there are three major types of cuts that are made during pruning. First, the large cuts are made to maintain the three to five scaffold branches (these cuts need to be made with a saw in a mature tree). Second, the tree can be topped at the desired height (with pruning shears). Then the pruner, again with the pruning shears, can thin the defoliated and crowded laterals and shoots. Most of these last cuts will be undercuts, where the downward-growing laterals are removed to lighten the rest of the branch.
In a sick tree, the canopy can be severely reduced to lessen the load on the ailing root system. An under-vigorous tree should not be asked to support a crop while it is rebuilding its vegetative capacity. Olive trees should not be pruned in the rain as the open wounds provide an entry point into the tree for infection. If you are pruning a tree with any sort of infection, be sure to clean your cutting blades with alcohol.
Consultations for site-specific pruning recommendations are available. For more information or to schedule an appointment, visit our Consultations page.