Dormant Oil and Pruning Fruit Trees

This is a tough year.  With dry, warm weather some of our trees are confused.  Many of our fruit trees never really hardened off as reported by our local research station in Grand Junction.  And now many of the buds are swelling on trees in the Grand Valley.   I always tell homeowners to wait to prune their fruit trees later than the commercial orchards so they are closer to spring for the trees to heal.  Growers have to start early due to the sheer volume of pruning they need to do.  And I prefer to spray dormant oil after pruning but this may be one year I decide to do the opposite on my own trees.

Colostate photo-Our goal-Fruit!

I think everyone that growers fruit trees should spray with dormant oil.  This product used to be made from petroleum products but is now usually made from cotton meal seed so it can even be used on organic trees.  Yes, organic trees are sprayed, but that is a whole other article.  This oil is very thick and is only used in winter versus horticulture or summer oils that can be used during the season.  The reason we spray this is to suffocate any insect eggs or overwintering insect adults on the trees to limit pest pressure on the fruit tree in the coming season.  It is an especially good control of aphids.  Make sure the weather is warm enough to spray and that your buds have not broken (not showing any color).  The oil can burn the contents of the bud if open.  Always read the label, it’s the law.  And be sure to cover as much as the tree as possible.  Here is our fact sheet on dormant oil.  It can also be a good control for some insects on ornamental trees as well.  http://extension.colostate.edu/topic-areas/insects/insect-control-horticultural-oils-5-569/

So why do we even prune fruit trees?  Well of course to get better fruit.  Pruning a fruit tree is vastly different than pruning an ornamental or shade tree.  Many of those rules go out the window because with fruit trees you are actually trying to stress them to induce fruiting.  So our goal is to capture enough sun in the fruit tree canopy without sun burning the fruit.  The best shape to do that would be an upside down umbrella.  Now if you do not want to have a tree that looks like an upside down umbrella in your landscape, you can prune to have a central leader with spaced side limbs.  This is an older method of pruning apple trees and cherries also do well with this system of pruning.

Before pruning different types of fruit trees it is important to understand what year wood gives you the fruit.  With peaches the fruit occurs on one year old wood, the new growth from last year.  On apples, it is typically 2^nd to 4^th year wood but some depends on variety.

With pruning these trees we want to remove 50-60% of the tree if it is young and used to being pruned that way.  Don’t take an old tree and do this, it will stress too much.  Talk to your local agent if you have an older tree.  Here is a link: http://planttalk.colostate.edu/topics/fruits/1210-pruning-mature-fruit-trees/

Open Center Peach Pruning- Photo by Susan Carter

So young trees have their central leader removed when they have established root systems.  Here is a a diagram from UMN. 

Now back to in their prime trees; when pruning you want to think of the side shoots of the limb together looking like a feather.  So there are very few branches going up or down.  Branches underneath tend not to get enough light and too many branches going in or up can shade other branches and leaves.  But you want a few interior and upright branches to produce future growth and to provide some shade for your trunk and main branches or they may develop sunscald.  http://planttalk.colostate.edu/topics/weeds-cultural-problems/2111-sunscald-trees/

There are different types of cuts used to prune fruit trees.  My favorite to help keep the limbs going out instead of up is called a bench cut.  This cut removes the tip of the upward growing branch and brings is back to an outward facing side branch.  This side branch will then become your growing tip and will help keep the height of the tree down and to create that upside down umbrella shape.  Dutch  cuts are vertical cuts that are made on side limbs of mainly apples to induce dormant bud break below.  Something you would never do on an ornamental tree. 

Apple tree with dutch cut- photo by Susan Carter

So to summarize, start pruning and training your fruit trees when young;  Spray with dormant oil prior to bud break;  Don’t  be afraid to remove half the tree yearly;  Know the age of the wood that produces fruit so you don’t remove too much of it. 

Lastly, look for a local training on fruit trees thru Extension.  The Tri River Area offers a fruit tree class as part of their Master Gardener training.  I promise a future article on picking which fruit tree is right for you or should you support the orchards and let them do the labor.  By Susan L. Carter, Horticulture Agent, Tri River Area