Stressed Trees: how to prune and care for them

         Trees have it hard in Colorado. If it’s not the shallow and lean topsoil, it’s the low rainfall and low humidity, or it’s the heavy wet late spring or early fall snows, or like last November, it’s the dramatic temperature changes. After a warm and beautiful fall without a killing frost until November 11, we experienced a 77 degree drop in temperature between November 10 and November 12. This was one of the three largest temperature drops ever recorded in the Denver area, the other two were in December 2013 and January 2014.

 This record is significant to trees and plants because they need time to adjust to cold. They need to harden-off, to form a protective layer between the leaf stem and the branch, and to move the sugars from the leaves down into the stems and roots to store food for spring growth, flowering and fruiting. So now we are seeing the effects of freeze damage and stress especially in evergreens and broadleafed evergreens. We will see dead tips and dead branches in the deciduous trees and shrubs once they have leafed out. And there could be extensive damage if the leaves that did not fall off have left openings into the plants for fungal diseases to enter. This happened around 1990 when major portions of Siberian Elms died off due to fungal infections.

However don’t be in a hurry to cut out unsightly dead branches, because coming out of dormancy can be delayed by shocking conditions. There are a couple tests you can use to see whether a branch is dead or just slow to leaf out: use a finger nail to scratch a little bark off and look underneath. If it is green, it is alive; if brown (and wrinkled) it is dead. And press against a bud; if it is flexible, it is alive; if it is dry and breaks or crumbles, it is dead. If in doubt, wait and see.

Once you are sure a branch is dead, it is best to cut it off because dead material is what attracts the decaying fungi. Decay is great in a compost pile, but  decay in a  shrub or tree branch can spread down into live wood and weaken or deform the structure. And so it is essential that you make the pruning cuts properly, because bad pruning can make the problem worse.

The late Dr. Alex Shigo, who spent his lifetime dissecting, researching and watching how trees respond to wounding and pruning, discovered the “branch collar”. He showed that if we cut through the protection zone represented by this swelling at the base of most branches, decay will more easily enter. Some people, including teachers and professionals, believe that leaving the branch collar means leaving a stump or stub. This is not correct, because the shrub or tree cannot grow over a stump and it will leave an opening into the plant for years that can lead to decay or diseases.

The illustration shows the cut D as one that leaves a stump, the cut A-C as a flush cut that removes the branch collar and the protection zone. The cut A-B is the correct place to cut that leaves the branch collar. It starts just outside the bark ridge and slopes out slightly so as to leave the swelling. Notice there is no stump remaining after this cut. The other illustration shows the decay that follows a flush cut, on the left, and the smaller decaying area from a correct cut on the right. Trees and shrubs do not really heal like animals, they wall off decay so that it cannot spread, but the old tissue is not renewed. Therefore we should not cut off the branch collar and the protection zone.

When the dead wood has been removed, then other helpful pruning can be done. Thinning cuts can remove rubbing or crowded branches. Branches need room to grow and they need light for photosynthesis and for good air circulation and health. Some larger branches may need to be thinned, but cuts close faster and there is less suckering response when thinning is done in the smaller wood.

After storm damage or freeze damage, and subsequent removal of broken or dead wood, a shrub or tree may be misshapen, out of balance, or just plain ugly. The branches that were not broken or dead will grow faster and sprays of sucker growth often grow from the large cuts. This can be corrected, but may require two or three prunings over as many years. It is good to stand back from the tree or shrub and view it from several angles. Like any sculpture, it should look good from all sides. Beauty and balance and proportion are connected. So after the damage is removed, if the resulting shape is one-sided or awkwardly off balance, shorten the long branches by 10%-20%. For optimal structural strength, the tallest branches should be positioned over the trunk. When shortening a branch, it is important to leave a “leader” or side branch at the very end. This leader should be no smaller than a third of the diameter of the branch where it is cut.

Stressed and damaged trees and shrubs also appreciate some other support. If natural rain or snowfall is not enough to make the soil moist, water deeply every 10 days to 2 weeks. This watering can be combined with mild fertilization. You may have heard that it is not good to fertilize a stressed tree. I believe that recommendation is only valid for chemical fertilizers. Our soils are not naturally rich. Most are deficient in organic matter and nitrogen. In addition, our alkaline soil pH reduces the availability of some minerals. Because weakened plants regrow more slowly and are more vulnerable to pests, I would suggest broadcasting a mineral supplement and an organic fertilizer, at half the usual rate, over the root zone and watering it in slowly over a long time. There is no value in watering near the trunk since most of the absorbing roots are located in a wide area near the drip-line (the ends of the branches). You can also use a yucca extract, like Sledgehammer, which breaks down the surface tension of water improving absorption of water and nutrients into the roots and through the cell membranes. 

It can be discouraging to see a lot of dead wood and an ugly form, but if the plant has a healthy root system, it can regrow and recover much faster than another new plant. Correct pruning will provide the training. If the tree or shrub is too old and decayed with a weak root system, is diseased, split or just doesn’t have the strength to come back, you will know by mid-August/September, which is good time for planting.

Mikl Brawner was an arborist for 35 years. He and his wife, Eve, are co-owners of Harlequin’s Gardens, which specializes in organic veggie starts and herbs, natives, sustainable roses, xeriscape, unusual perennials, and products to build healthy soils.