I saw a group of trees that have been topped at the Garden, but I thought that topping trees was bad for them.
— Angela Groskopf, Park Ridge
You may be referring to the group of sycamore trees planted to shade the amphitheater at the Regenstein Fruit & Vegetable Garden. They were recently pollarded, which is a method of pruning. Providing shade for Garden visitors is an important design consideration for this space while also having the trees limbed up high enough to preserve sight lines for guests watching demonstrations in the kitchen. The space here is limited, so keeping trees from outgrowing the space is an important maintenance consideration. The Garden decided to use pollarding to maintain these trees to meet the design criteria and to provide an example of pollarding, which is a practice more commonly found in Europe.
Pollard tree pruning is a method of trimming trees to control their size and shape. The technique is generally used on trees planted in an area where they cannot be allowed to grow to their full size. Over time, pollarded trees develop a unique, twisted look to them during the dormant season. In summer, they tend to have a rounded shape as they resprout. Pollarded trees provide ample sun in the winter once they are pruned and abundant shade in the summer as their new growth fills in. Not all tree species are suitable for pollarding. Trees such as sycamores and lindens that grow large and sprout readily after pruning work best.
Pollarding starts when trees are young. The pollarding of the trees at the Fruit & Vegetable Garden started about five years after planting, with careful and strategic initial cuts. The central leaders of the trees and all lateral branches were cut back to the same general height within a few feet of the tree crowns. Any crossing limbs were also pruned to provide a good structural foundation.
A pollarded tree will produce new shoots at the site of these cuts, which should be pruned off once a year during the dormant season. The new shoots are always cut back to the same place, leaving the larger branch intact. In time, the place where the original cut was made becomes knobby due to the tissue formed at the base of the shoots. The placement of the pruning cut each year is very important for this technique to be successful. Position the pruning cut to avoid leaving a stub while leaving the knob intact, because that is the source of the following year’s growth. Every time the water sprouts are pruned off, the tree is able to heal the wounds and resprout from dormant buds in the surrounding tissue the next year. Topping is completely different from pollarding. Topping often removes large portions of the crown of a tree, resulting in wounds that a tree may not be able to heal properly and new growth being poorly attached and a risk for failure. Topping trees does not lead to the formation of knobs at the ends of the branches.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at [email protected]. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.
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