Pruning to give spur leaves light

alan haighJune 11, 2014

I was recently reading an article in Good Fruit Grower magazine about the research of Alan Lakso who, during a long career at Cornell U., studied apple tree physiology to determine best pruning and other management strategies to get highest productivity from apple trees.

A couple of his observations and discoveries may be of use to those of us called upon to prune apple trees, especially vigorous, free-standing ones (M7 and up).

First of all, spur leaves, the small rosette of leaves surrounding fruit clusters, devote all their available energy to the fruit in their center. Nearby shoots do not, and only send their energy forward to new leaves and wood until they stop growing- sometime around mid-summer. It is only at this point that they send it back to help support fruit, wood and root behind.

I think his most important discovery was that after a certain period of being shaded from the sun, spur leaves lose their ability to photosynthesize. If you summer prune to expose them to light after they've been shut out they will be unable to supply the fruit with any energy. He recommends that you avoid this atrophy by pruning the trees open enough so the spur leaves are never excessively shaded by annual shoot growth.

The most productive orchards have a higher percentage of light reaching the spur leaves- least productive have more light harvested by growing shoots.

It seems to me, that, for the home grower, it might be well worth it to do some spring pruning to maintain a high level of light on the spur leaves. Commercial growers tend to only prune once during the growing season, usually in mid-summer to help apples get color. Earlier pruning would increase labor costs because of regrowth.

It seems possible, however, that maximum size, quality and productivity could be encouraged by making sure spur leaves get optimum light throughout spring into mid-summer by removing a percentage of the shoots that are creating shade- perhaps leaving the less vigorous shoots to nourish the fruit after mid-summer and for spur renovation.

I just finished opening up the apple trees in my own orchard- just a few days after completing thinning of fruit (it's been about 3 weeks since petal fall of last apples). I know now that all the spur leaves are getting plenty of light. I'm curious if this will help me defeat biennial bearing on a few of my less cooperative trees. I expect it will.

I also suspect that other species tend to operate in a similar manner, with growing shoots sending most energy outward once they've reached a certain distance from the fruit. I'd love to see more research on this general topic as it applies to plums, pears, peaches and other common fruits.

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I watched a u tube video on how to prune cherry trees. They had some guys from the university teaching a class. He said the first step is to remove all weak and pendent wood, wood that is pointing downward or under other wood. He says that those branches with fruit have to get their carbohydrates from the next part of the branch in the sun steeling it from that fruit to make the fruits on its branch. You get sweeter bigger fruit by removing the fruiting wood that's pendent. He says to get vigor back into the tree by making big cuts on big wood they have to be in the sun or it wont send out the new growth. These professors talk about how many leaves going up the limb are responsible for the cherry clusters carbohydrates so part of what you are saying is the same with what they are saying with cherry, but they are also saying it's stealing it from another.

    Bookmark   June 11, 2014 at 11:04PM
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fruitnut Z7 4500ft SW TX

I'd like to read more about light levels associated with the loss of photosynthetic ability and also about shoot contribution to fruit. This is the first I'd heard that leaves could permanently loss their ability to function by being shaded for a period of time.

    Bookmark   June 11, 2014 at 11:48PM
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Harvestman. This is a good read. Looks like my window for this type pruning is past for this year. Please post your conclusions later this season. IâÂÂm assuming but not sure if this technique would also work on my pear trees. My pears are approaching 1.5-2â now and I was going to do the summer pruning soon. If your results are positive, I may give it a try next spring. Thanks, Bill

    Bookmark   June 12, 2014 at 6:33AM
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Scott F Smith(6B/7A MD)

Interesting stuff, hman. I have a bunch of very closely spaced apples in 6-hour sun and have implemented a pruning plan that more or less achieves what the article suggests by how I have thinned out the scaffolds and thinned the size of the fruiting plane. All the fruit I try to get in a 4-6' zone off the ground, a bit like grapes, and it makes it easier to get sun on all the clusters.

What I like about this particular article is its a good general guideline for pruning: if clusters are being overly shaded, get that problem solved yesterday!


    Bookmark   June 12, 2014 at 1:03PM
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alan haigh

I found the actual study and I was misled by the GFG writer or misread the writer in a very important way. Shaded spur leaves don't entirely lose their ability to photosynthesize- only a percentage of it. The discovery was that once this percentage is lost by shading exposure to sun does not help it return.

Anyone want some wonk- here it is.

Here is a link that might be useful: Lakso study

    Bookmark   June 12, 2014 at 6:12PM
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Hummm, unfortunately Lakso's research papers are behind an "Acta Horticulture" paywall.

    Bookmark   June 12, 2014 at 9:52PM
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alan haigh

FascN,this is not a political forum. If your statement is related to home growing fruit or if you can even make an argument about how this corporate connection has anything to do with this studies conclusions, as being discussed here, please explain.

This study was done years before it was discussed in GFG magazine, but I'm actually curious how you see a relationship between the interests of the advertisers (ag chem corporations, fruit tree nurseries, ladder makers and the rest) and a study about the affects of summer pruning.

The study's conclusions only suggest training trees more open (more labor)- not to use more thinning chemicals. Chemical companies benefit from tactics that involve less labor and more chemicals.

Oh, I get it- it's about selling more ladders to pruning crews. Those evil ladder manufacturers!

    Bookmark   June 13, 2014 at 5:13AM
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emmers_m(9a/Sunset 7 N Cal)

I think Facist_Nation's point is that you have to pay to read any of the research papers published by Acta Horticulture - the magazine.


    Bookmark   June 14, 2014 at 10:02AM
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alan haigh

Man, it was the name Fascist Nation that made me so presumptuous, and also the fact that the study is available to read for free- The link gets me there, did it stop you guys?

I used to be a member but haven't renewed for a couple of years so I'm surprised I get through if others can't.

My apologies Fac. Nat. Now I've learned a new word- paywall.

    Bookmark   June 14, 2014 at 11:00AM
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