Pruning & staking young olive trees for mechanical harvesting

This article is designed to assist growers in the pruning of their olive trees during the first two years after planting. The information given is for trees between approximately 1 ft and 5 ft in height. During this fast growth period the trees require specific pruning to maximize their growth, keep them in good health, and very importantly, prepare them for mechanical harvesting. The briefness of this sheet cannot give all the answers and options but it does give a basic guide to pruning and staking during the first couple of years.

Whenever you are pruning a young olive tree there are four main points to keep in mind:
1. Too much pruning at a young age will stunt the tree’s growth.
2. You ultimate goal is to prune for mechanically harvesting the crop
3. A central leader trunk will assist growth in the early stages.
4. Practice makes perfect!
Let’s take a closer look at these points.

1. BODY TALK – Olive trees are like human beings in many ways and in no way are they so similar as in the pruning. It’s as simple as this:

A human can afford to lose an arm or even a leg and still live reasonably well BUT if you lose both arms and both legs at the same time, you’re in trouble! – SO IS AN OLIVE TREE.

If your young tree is 3 ft tall and has side branches growing all the way up its trunk DO NOT take them ALL off just because you’ve read that you need a clean straight trunk for 40-48 inches.

At first, only take off any that are growing below 1 ft and then in several months time when the tree has grown considerably more on top, and has ‘recovered’ from the first pruning, you can take off any branches between 1-2 ft. Repeat this process until finally after about two years, you have your clean straight trunk for 40-48 inches. Don’t make the mistake of pruning ‘too much too soon’. This can send the young tree into ‘shock’ and set it back by up to a full year. Always leave large amounts of leaf on the tree for photosynthesis to take place so that maximum root growth etc will occur.

2. Mechanical harvesting is the most efficient method of removing fruit from olive trees. Whereas oil olives have been the only mechanically harvested olives for many years, table fruit are now sometimes dropping into catching umbrellas in countries around the world, including California. Unless you have made a clear decision to hand harvest your fruit, to neglect pruning for future mechanical harvesting in the modern orchard may be a serious error from a long term economic viewpoint.

So what shape of tree do we need for mechanical harvesting? The most important requirement is a straight trunk for 3’4″-4 ft from the ground. This section of trunk must finally be free from all branches to allow the harvester’s head to securely grip the trunk without any obstruction. This will allow the harvester to work more quickly and will also avoid damage to the tree.

Achieving this straight clean trunk occurs over about 18 months to two years. Initially, when the tree is only 1-2 ft tall you simply need to remove any branches which want to grow off the trunk below 1 ft from the ground. Other branches above 1 ft can be left to grow or, if they try to grow larger and faster than your main leader, they can have their tips removed to slow down their growth – thus allowing more nutrients to be focussed into the leader. This clearing will make it easier for weed spraying and will also allow the tree to focus all of its growth into the main ‘leader’ trunk and some higher lateral branches. At no stage should any growth touch the ground. In the early years, it is important to leave as much growth as possible on the tree because foliage promotes root growth which in turn promotes the production of more foliage.

Some trees will develop with a ‘leader’ (trunk) going straight up the center with small side branches. Others will head straight for the sky as a single trunk with no side branches. Both cases are fine, but with single trunks you will need to nip off the growing tip at about 48-50 inches to encourage side or lateral branches to grow at this place. It is these lateral branches that will form the main structure of your mature tree. (For notes on another pruning method see our page on Monoconical Pruning)

Mature olive trees need to be kept reasonably open in the center to allow light penetration for better tree health and fruit production. This is best achieved through a vase shaped, sturdy growth habit which also facilitates mechanical harvesting. Your trees will probably have quite a number of lateral branches at about 40 inches or so from the ground when the tree is 18 months of age. Thoughtfully choose out four evenly spaced lateral branches. These need not all come from exactly the same height but should not be any lower than 30 inches from the ground. As these will form the vase framework for your tree, if possible choose branches that are growing at least 30 degrees up from horizontal. This will give a vase rather than a flat plate shaped tree structure. Remove the other growth as outlined below.

If your main leader is damaged or slow growing for some reason then you may choose to allow a faster growing side branch to become the new leader. Simply remove the old leader from the stake and tie the new leader to it. (A bit like politics!)

When the tree is between 3-4 ft tall, and if it has plenty of leafy branches towards its top, you can remove the branches which are growing from the trunk between 1-2 ft from the ground. You should now have a tree with a straight clean trunk to 2 ft and a nice number of branches above 2 ft. If your tree is over 4 ft high then you can remove the tips of any branches that leave the trunk between 2-3 ft. (Don’t forget BODY TALK’s advice – if there aren’t many branches between 3-4 ft then don’t cut too heavily at this stage).

Several months after you have done the last step of pruning above, you can remove any of the final branches up to about 40 inches. Your trunk is now clean to the desired height for machine harvesting and yet you still have about four evenly spaced solid branches at the top of the tree to keep root growth to a maximum. Depending on variety, land preparation and climate this whole pruning cycle from a 1 ft tall tree to a solidly trunked tree which is branching well on top should take about eighteen months to two years.

3. A central leader trunk will help to speed up your tree’s growth during the early years. Because this trunk will be fast growing and always growing upwards in the center of your tree, it will be drawing nutrients up through the tree to sustain its growth. As it draws these nutrients up the tree, the nutrients will be carried to lower branches and thereby increase their speed of growth as well. The central leader acts as a type of ‘nutrient pump’ within the tree. (If your tree decides to grow straight up without any lateral (side) branches, nip the growing tip out when it reaches about 4 ft. This will force it to start lateral branching into your vase shape.) Don’t get this central leader method confused with monoconical pruning.

What we have finally achieved is a young tree with approximately four main scaffold branches. The shape is commonly known as an Inverted Conical Vase.

As mentioned in the introduction, this is not a conclusive pruning guide. It only touches on the basics of pruning young trees with what are considered the most commercially viable methods.

4. Practice Makes Perfect! Olive trees have a mind of their own and as such they will sometimes fight against many of your efforts to prune them into shape. Don’t give up. Perseverance wins the race. Remember that time is on your side. A tree that won’t grow correctly this season can often be restaked and then pruned into shape next season.

The staking of young olive trees is very important. Stakes need to be strong enough to support the tree while the anchor roots are developing, and yet flexible enough to allow the tree to move in the wind. If the stake is too rigid or the tree tied too tightly to it, then the tree will be over protected and not feel the need to develop strong roots. The small bamboo stakes supplied with the trees from the nursery will only support them for a short period.

Santa Cruz Olive Tree Nursery has been recommending the use of bamboo stakes for sometime. The most common size to use was the 5 ft high, 1/2 inch thick stake which is a very suitable stake. However in windy areas, about 10% of the stakes may need to be replaced in the first two years due to breakage. We have now found a thicker bamboo stake that will reduce this problem. This new stake is 5 ft high, 5/8 to 3/4 inch thick and considerably cheaper than hardwood stakes.

The bamboo stakes have some advantages over hardwood stakes as they have a smooth surface and do not damage the bark or rub out newly forming buds, which are going to be the scaffolding branches of the tree. They also allow the tree to flex in the wind thereby encouraging the tree to develop a thick trunk and also to develop its anchor roots quickly. A tree rigidly tied to an unbending hardwood stake will not realize the need to develop its anchor roots strongly. The bamboo stakes are also easy to handle, eliminating splinters and the lifting of heavy bundles.

These thicker stakes will last up to two years at which point the tree will no longer need staking. It is rare for a tree to need any support after it has outgrown such a stake. Place the stake about 2 inches from the base of the tree and push it into the soil at least 1 ft vertically until it feels quite stable. Another good option is to use the stakes to mark your tree sites prior to planting and then simply plant the trees beside them. You can then tie both the young tree and its small nursery stake to the main stake with a tool such as the taper described below. (There is no need to cut off the tapes between the young tree and its nursery stake as they will break away naturally as the tree trunk thickens.)

After planting and staking the tree, the stake will prove to be a good solid anchor point to attach protective guards or netting to if you have severe animal problems and do not have a full netting fence around the boundary of your orchard.

From our experience with tying methods over many years, we have found that the tape tying tools available from McCalif Growers Supply are an excellent investment. The tape tool is very fast and efficient and if you have a number of trees to tie, you will get the cost of your tool back very quickly in saved time. When you order your tool, generally the “Max Tapener” is the most popular choice. You will also need to ask for a packet of staples, a packet of spare cutter blades, and unless you have very thick trunks, the 100 ft rolls of tape will be what you’ll use.

After testing many brands of tape, we recommend the high quality, green, Italian tape which we now stock as standard. As the tree trunk grows thicker it will be better able to support itself without so much need for the stake. As the trunk thickens, the tape will stretch and naturally tear out at the staple point so it will not strangle the tree as some ties do.