I'm a homeowner in the NE USA and have a thirty foot ash tree in my yard that has survived neglect while I fixed the house and is finally being appreciated now that I have started to fix the yard. It is fairly healthy, a few upper branches have died but many produce new leaves every year and one of the two trunks, the right hand, is producing new side sprouts directly through the bark in many places. I want to encourage the left trunk to join in the fun and also produce new sprouts, preferably high up on the left side so in a couple of years a nice branch hangs across and acts as an informal screen between the high window of my neighbor's house and my yard so I get more privacy. Suggestions for encouraging sprouting are invited.
The only sure way to encourage sprouting is to remove some or all of the tree above the trunk area where you want the sprouts. You could start with the removal of a few upper branches to see how severe the pruning needs to be. Allowing more light to reach the trunk will encourage sprouting. A not-so-sure way is to girdle the main trunk just above the desired level. A narrow (around 1/4" wide) strip of bark and phloem should be removed completely around the perimeter of the trunk. I don't know how an ash tree will react to girdling; but I've done it on several types of fruit trees, with no adverse affects.
Thank you, your three suggestions are very helpful. Pruning at 30-40ft up probably requires a bucket-lift and is not a DIY project so involves some expense. On the other hand your point about light encouraging sprouting seems very relevant in this case because in early June I cleared a lot of climbing vine and forsythia from both trunks, in places the coverage was quite dense which probably blocked light from the trunks. Six weeks later there are new sprouts on one of the trunks and it may be that removing vines made the difference. I am encouraged to allow some time to pass in the hope that the left trunk will also sprout now it has been uncovered and is getting more light, this may do the trick. Regarding girdling I have two questions. If I understand correctly two cuts are made all the way round the trunk separated by about 1/4inch and the material between the cuts removed. Perhaps this might be done simply using a chain saw in a single cut. Is it necessary to control the depth of this cut? and if that were attempted by someone with no tree care training what might be the risks of an adverse outcome?
Don't use a chain saw for girdling! You would probably damage the trunk excessively. Girdling is usually done with a special girdling knife, but I just use a very sharp kitchen knife and make two parallel cuts approximately the right distance apart. The depth of the cuts is controlled by the thickness of the bark/phloem layer. You cut to the bottom of the phloem where the wood becomes much harder and the cut out material slips easily off the cambium underneath. I've always done this in spring or early summer; the phloem layer might not slip off as easily in other seasons of the year.
I don't want to be the skunk at the party, but I have to be suspicious of your Ash that suffered from neglect. Mature trees don't usually have a problem with being left alone for a few years. This is not to say they don't need a little tending and maintenance, but it sounds like you have more than that. Do you have an Emerald Ash Borer problem in your area? You might want to check with your county or city natural resources agent. One phone call could be helpful. In Michigan, we have learned the same old lesson of why street trees should be as varied as possible rather than planting miles and miles of hosts waiting for customers that can conveniently fly ten feet to the next host...
