Olive Biology
The chapter has the inevitable disadvantage of being potentially the most enriching, yet the most challenging material to tackle. 'I know the meristematic apex of the bud is important to olive flower bud induction,' the man says, 'But what the heck is a meristematic apex?!'1 And just because we we can dream up a word for something doesn't mean we understand it, just that we observe it happening and may or may not be amazed. Like a good magician. And yet, the life cycle of a plant is amazing even if we merely tip our hat to the science and pour fresh olive oil over our garlic-rubbed piece of toasted bread.The most important thing to remember about olive tree biology is that, at any given time, there are two crops developing on the same tree. This years crop shall flower, be pollinated, develop fruit and ripen into commercially viable olives. At the same time, next year's crop is being formed by the new buds in the vegetative growth which are the progenitors of the succeeding year's olives. The more energy the tree expends this year to fruit bearing, the less it will have for shoot growth and vice-versa. The good news is that we can influence this with fertilization, pruning, judicious irrigation and prudent harvesting techniques. The bad news is that an olive tree, like many other types of trees (apples or pecans, for example), has a natural tendency to bear fully every other year (alternative bearing).
You can compare olive trees with nursing mothers. A woman has a much lower probability of becoming impregnated during breast feeding if her nutritional losses are not offset by substantial additional nutrient intake. Thus, the poverty stricken third world mother can nurse her child until the age of four or five and be fairly certain she will not become pregnant during this period. Nature’s way of family planning. For a first world mother who has the luxury of altering her dietary habits at will, this is not an issue and she must fool her body in other ways to avoid becoming pregnant. An olive tree is the same: Any practice that diminishes olive tree vigor adds to the burden and extent of alternative bearing. Examples of this would be a deficiency in soil nutrients, drought, leaving a large crop on the tree too long, disease or pests and weather extremes. In short, both the mother and the tree are
smart enough not to put out progeny when the conditions are detrimental to their offspring.2 And it appears that they learn to error on the side of caution as they get older and gain more experience about the fickle ways of the world.
Fertilization
Traditionally, horticultural knowledge was based solely upon observation: Fertile soil increased the performance of the trees and soil was made fertile by the addition of some sort of organic matter. The same was true with tree pruning: A cut tree grew more olives. Olive trees bear less every second year. Old oil is bad oil, blah blah blah. Modern science has, however, given us distinctive advantages because, by understanding the biomechanisms, we are in a much better position to more precisely manipulate them. Suddenly the ‘hows’ are less important than the ‘whys’.Which brings us to photosynthesis and the nitrogen cycle. Two riveting topics which cannot be discussed in enough detail. Like humans, an olive tree needs both carbohydrates and proteins and essentially for the same reasons: energy and growth. And like humans, the olive tree takes in its nutrients through chemical processes driven either by solar energy and/or absorbed directly from the soil. These two processes are known as photosynthesis (the carbohydrates) and the nitrogen cycle (proteins) respectively.
Photosynthesis (photo=light and synthesis=to put together) is solar radiant energy used to convert carbon dioxide (CO2 - approx. .03% of the atmosphere) and water (H2O) into sugar (glusose) and oxygen (O2). This process occurs exclusively in organelles called chloroplasts (located in the leaves) which further requires the presence of the green pigment Chlorophyll.3 Simply put, the leaves (and only the leaves) are both solar collectors and a two-way valve to allow CO2 in and O2 out, the byproduct of said reaction, the released hydrogen (H), together with nutrients from the soil such as nitrogen, then provides energy for growth. Even more simply put, Photosynthesis is light splitting water.
The nitrogen (N) cycle is equally important and more easily controlled by man. I cannot make the sun shine, but I can rake up nitrogen rich organic matter and spread it around my tree (remember that every chlorophyll molecule alone has four nitrogen atoms).4 Were a tree left alone, it would replenish its nutrients without any problem because the fallen fruit, leaves, branches and any other tumbleweed that comes along will be like a protein cocktail. That is essentially the nitrogen cycle.
The problems begin in a commercial operation. By harvesting the fruit and pruning away leaves, shoots and branches, you rob the tree of its usual nitrogen resupply sources and cause a nitrogen deficiency. That becomes then the type of fertilizer the soil most desires. (There are essentially 14 different nutrients plants need, however, deficiencies in olives have generally only been noted in nitrogen, potassium, phosphorus and boron).
Nitrogen in and of itself, cannot be used by the olive tree and must first be converted into nitrates (NO3) by the nitrobacter group of bacteria.5 Together with ammonia, the nitrates are removed from their soil solution by the olive tree and utilized in photosynthesis to create amino acids which, ultimately, is the tree’s protein. Too many nitrates, however, would be toxic to the tree, essentially ‘burning’ it through oxidation (remember those three oxygen molecules hanging onto the one nitrogen). You might have seen this if you gave a flower too much fertilization. Too much of a good thing. Think of it as rust, a destructive reaction, or free radicals in mammals.6
How then do you know what your soil needs? The obvious first clue is the way your trees look. As I just mentioned, are they having a toxic reaction to something in their environment? Are the leaves discoloring or falling off? Perhaps there's a potassium shortage. Has the olive yield or shoot growth decreased suspiciously? Sounds like it might be a nitrogen deficiency. Are the olives deformed (monkey-faced)? Better check the boron level. Tissue (leaf) and soil analysis are yet more precise methods.
Irrigation
Give a man a fish and he has food for a day. Give him a fishing pole, a boat, six months experience with a knowledgeable fisherman from the target area, and he might, given weather conditions, environmental laws and fish market circumstances, he might just possibly make a living. The same is true with olive trees. They will gladly take as much water as you give them, the question is, however, does this make them lazy and/or weak? Yes.First we need to understand the role of water (H2O): Simply put, it provides the hydrogen for photosynthesis and the oxygen we breath. Water is truly the basic element of life. Unfortunately, plants can burn through water as quickly as a teenager under the shower. Even if they can soap and rinse with twenty liters, you can count on them using fifty. Photosynthesis stops due to biochemical limitations7 even while transpiration (the oxygen ‘waste’ product the plant exhales) continues at the same rate. Just like the teenager, we need to tell the tree to use less water because, even if we have adequate supplies now, we may not later.
The good news is that olive trees are considered drought tolerant. Their small leaves are thick and leathery with a waxy cuticle on the upper surface and hairs on the lower surface to limit transpiration (water loss). The stomata are located primarily on the lower surface in depressions which also reduces water loss. To kill an olive tree with drought is very difficult, but is survival the same as economically viable olive production? No.
Obviously water is important, but the olive tree is a hard-headed creature and to get the most out of it in its natural habitat, you’re going to have to practice tough love. It has to be forced to adapt to its environment rather than artificially changing the environment to suit the tree. For example, if you cocoon your children in a fantasy wonderland, the day will arrive, most certainly unexpected, when those kids have to face the world’s brutal realities. The trees, like children, need some nurturing - we certainly can’t expect babies or juvenile olive trees to fend for themselves, but we must not spoil them. Let them make do with what they have because odds are, they have enough.
Pruning
'He who plows the olive grove asks it for fruit; he who manures it begs for fruit; he who lops it forces it to yield fruit.' This old proverb cited by Columella goes to the core of why we prune: To force the tree to develop as we choose.But even more critical is to give photosynthesis a helping hand: We need to guarantee that sunlight, a critical need in every stage of the fruit’s development, is not lacking. It cannot be stressed enough: The olive tree bears fruit, with some exceptions, on one-year old wood found on its outer periphery in the presence of adequate sunlight. Once we’ve funneled the sun’s energy, then we need to make sure we are growing productive (i.e. olive bearing) shoots. Finally, once we have our olives, we want them in a place where we can easily get them off the tree. Imagine the difference between picking olives stretched out on a tottering ladder, bending branches (and thus breaking many future fruit bearing shoots) to get that last cluster or climbing into the wineglass-shaped bowl inside the tree after laying your ladder against one of these low branches with a friendly canopy to support and cradle you as merrily pick the tree clean. The same is true with tree shakers: You want a strong main trunk and enough clear space under the tree to get your machinery close.
The question remains, how much and how often should we prune? People are all over the board on this subject. A much cited standard is that the tree should be pruned so that a bird can fly through it without touching a leaf. Yet Mort Rosenblum, in his book ‘Olives’, cites trees producing 150 kg’s which could ‘hide a peacock in a dayglow T-shirt.’ (After citing the classical proverb). My short answer? There is none, just a list of guidelines such as, the greater the intensity of pruning, the greater the vegetative response and the longer the length of of the interval between pruning, the greater the intensity must be practiced. Pruning, however, is a skill which must be learned first hand and keep in mind that two experienced pruners might disagree on what branch should be cut.