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In Iowa, fruit trees should be pruned between late February and early Apil. I normally take a day off of work around the last week of February to prune our fruit trees. I don't prune when precipitation is forecasted within 48 hours or when the 10-day weather forecast has any temperatures below 0 degrees F and I've always found a date in late February that meets those requirements....until this year. I watched the forecast all February and early March and there wasn't a single day that met those two requirements. Fortunately, the weather in the middle of March has been much better so I took a half day off work and started pruning and just finished the rest of our fruit trees last weekend.
I prune to increase sunlight and airflow throughout the tree which helps minimize potential disease issues. While pruning reduces the number of fruit a tree will grow, the remaining fruit will be a lot bigger, more than making up for the loss in the number of fruit. Pruning also stimulates new growth necessary to maximize fruit production. Living plants like to balance top and root growth so removing some branches stimulates the dormant buds to grow in an attempt to restore the top to root balance.
I try to follow the principles found in "Grow a Little Fruit Tree: Simple Pruning Techniques for Small-Space, Easy-Harvest Fruit Trees" although I'm not perfect at it. I have pruned our apple, peach, apricot, and plum trees to an open-vase or goblet shape.
Pruning starts at the time the tree is planted by cutting it off at 24-30 inches above the soil and cutting any shoots below that to 2-3 buds. The next 2-3 years I focus on getting 3-5 main branches evenly spaced around the trunk of the tree and shaping those branches to 45-60 degrees so they will be able to carry a heavy load of fruit. This may sound easy to do in theory, but first-time orchardists may find it emotionally challenging to remove unnecessary branches from a new tree, but the tree will be much better if it is done early instead of waiting a couple of years and the branches start to crowd each other.
Here is the 10 step the process I go through for each of our apple, peach, apricot, and plum trees once I've determined the main branches:
1. Sterilize hand pruners and hand saw. I typically fill a small cup with rubbing alcohol and let my hand pruners sit in it for about 10 seconds before making any cuts. I wipe my handsaw down with hand sanitizer and let it sit for 30 seconds before wiping it off.
2. Remove all dead wood.
3. Remove all diseased wood (sterilize pruners or saw again after removing these).
4. Remove all damaged wood. Damaged wood is frequently caused by branches that cross each other, or branches that rub against the support posts when the wind blows.
5. Remove all branches pointing to the center of the vase. These block the sunlight and will be hard to reach as the tree grows larger.
6. Remove all branches pointing straight up. These will not fruit and they shade the branches below them.
7. Remove all branches pointing straight down. These will be shaded by the branch above them.
8. Look for branches that are nearly parallel to each other or have a narrow branching angle. Remove one of the two branches leaving the one that most appropriately fills the remaining space.
9. Remove 1/3 to 1/2 of the new growth on each remaining branches by making a cut about 3/16" above an outward facing bud. Make the cut at about a 30-degree angle.
10. Remove all suckers growing from the rootstock. These are not the same variety as the main tree and should be removed whenever you see them.
While going through the 10 steps above, I keep in mind that I should not remove more than 30% of the wood (25% in more mature trees) in any one given year. Removing more than this in one year will send the tree into panic mode and it may try to regrow with suckers and water sprouts (rapid growing stems that grow straight up shading the tree and don't bear fruit) which can continue for a couple of years.
If all of this seems daunting, I'd say there is no one-way to prune a fruit tree. If I had 5 professionals look at a tree, I'm sure they'd give me 10 different answers on how it could be pruned correctly. The key is to do something and if it doesn't turn out as you want, next year you'll have another chance.
As with so many things, having the right equipment is half the battle. The first couple of years after planting our first trees I used a few different brands of hand pruners from the local big box store and didn't like how they felt in my hand and really didn't like the quality of cut on the tree. A couple of years ago I bought some Felco hand pruners and have been super happy with them (other than when my kids use them as scissors and don't put them away in the right place...but that has nothing to do with the quality of the tool!). I'm not a fan of using loppers for fruit tree pruning so I use a Corona pruning saw for everything bigger than what the hand pruners will handle.
One of the benefits of pruning trees is I end up with dozens of scions (the one-year-old growth that can be grafted to rootstock to make another tree) to use for grafting or to trade for other plants. I have connected with several people in a scion exchange group and have traded scions for several fig, elderberry, and aronia berry cuttings so we can expand our backyard plantings even more. I'll swap the rest of my scions with a local friend who will take them to a grafting class for dozens of other people to use in making their own trees. It is fun to think that by next year, scions from my backyard will be growing as baby trees all over central Iowa, plus Minnesota, Indiana, New York, and Kansas.