This morning I had this email exchange with a customer about pruning trees to a single stem before applying tree tubes. Since I have some version of this exchange/conversation several times per week I thought it would be helpful to reproduce it here:
Customer question: In response to my frequent advice on this web site to prune seedlings to a single stem before applying tree tubes the customer wrote: “I’ve always been reluctant to prune much of anything the first few years of growth of my newly-planted stock (I usually plant 1-3′ bareroot stock), thinking those trees/shrubs needed all the photosynthesizing power they could muster the first few years to get growing.”
My reply: On the pruning of seedlings… I am highly in favor of pruning to a single stem before applying the tubes, than pruning periodically as the trees grow inside. I have come to this position gradually and grudgingly, but as with everything tree tube related I will probably be proved wrong.
My urban forestry training taught me that you should never prune more than 1/3 of the live crown at a time, and even pruning that heavily stresses the tree. Common sense tells us that every leaf is a little solar panel providing photosynthate to fuel growth. So I completely understand the reluctance to prune such little trees so heavily.
10 to 20 years ago I generally recommended pruning to a single stem before applying tree tubes. However, my reason for this was that at that time most tree tubes were used on large scale forestry or CRP plantings where the trees would receive little to no aftercare, and I didn’t want the trees developing long terms with lots of very narrow branch crotches that would form very weak connections down the road. In other words I thought I was asking people to sacrifice some growth (by pruning off a good chunk of the “power plant”) as a trade off for producing a tree with better/stronger form. For folks who were working on a smaller scale and were willing/able to lift the tubes and prune my recommendation was to not prune in the first year or two, and then come back in year 2 or 3 and prune the lower branches as the tree added more growth to the top.
Then I did some trials where tested it both ways – leaving trees un-prune and bending the branches upward to fit the tubes (intending to prune these low laterals in a couple of years) versus pruning to a single stem. Much to my surprise, those I pruned to a single stem outgrew those I left un-pruned… significantly.
I have come to think that there are 3 reasons for these results:
1) It’s not a matter of each leaf only producing a fixed/finite amount of photosynthate. The more leaves in the tubes the more they shade each other, thereby reducing the output of each leaf. So a tree pruned to a single stem with (for example) 12 large leaves might achieve more net photosynthesis that a multi-stemmed tree with (for example) 24 leaves that are shading/crowding each other.
2) I have come to believe that air movement is critical in the tubes. I have seem cases where leaf growth from multi-stemmed trees “plugged” the tubes an inhibited air flow (especially CO2 exchange, or so I surmise). The once the trees sent up a main leader above the “leaf plug” growth literally exploded. I think that un-vented tree tubes restricted access to CO2, and increased CO2 exchange and availability is the primary advantage of vented tubes, at least for broadleaf trees. I also think that lots of lateral growth also restricts flow of/access to CO2.
3) There is obviously an element of simply channeling growth into a single stems as opposed to multiple stems. However my belief is that in pruning to a single stem we are actually producing more biomass over the course of the season, and not just channeling the same amount of biomass upward.
Having said all this,
1) I am sure that there are many instances where the opposite holds true. Pruning to a single stem fits with the history of tree tubes in that it’s utterly counter-intuitive (in the same way that common sense suggests trees will “cook” in tree tubes, or that vented will create a drying “chimney effect”). But it could also turn out to be wrong, at least in some cases and with some species.
2) The idea can be taken too far. A friend of mine who is the best nurseryman I know followed the pruning to a single stem concept to its logical end point: He scraped off all lateral buds with his thumbnail leaving only the terminal bud. He found that this suppressed the growth of the trees, as compared to those he simply kept pruned to a single stem but on which he allowed leaves on the main stem.
Yes, I write very long emails… usually because there isn’t enough time to make them shorter. So email me at your own risk!