Temps putting stress on trees
By Roger Larson | 11th February 2008
One of the consequences of our “roller coaster” temperatures has been the stress it creates in trees. David Robson, Extension Horticulture Educator from the Springfield Extension Center, provides some insight into addressing this stress.
“Frost cracks, separation of bark and wood in trees, are likely upshots of the recent intense cold in December followed by the unseasonably warm spells in January.
Frost cracks most often form in periods of wide temperature fluctuations, such as those we have experienced this January, he says. The openings may be large enough that you can fit your hand into the wood. The splits may go in to the center of the tree or beyond.
Trees that develop frost cracks are susceptible to repeated splitting and can become infected by cankering or wood-decaying fungi. Frost cracks aren’t really due to frost, but to drying and shrinking of the wood.
When trees are exposed to freezing temperatures, water leaves the cells and freezes in the spaces between the cells. This freeze-drying results in drying of the wood in much the same way as green lumber dries and cracks when exposed to the sun.
The cells shrink, and the tree trunk tries to shrink as well. But at the same time, the temperature in the center of the trunk remains much higher, and little drying or shrinking of wood occurs there. The unequal shrinkage between outer and inner layers of wood sets up a great strain that is released only by the separation of layers.
The break happens suddenly along the grain of wood, and it’s usually accompanied by a loud “crack” that can be as loud as a rifle shot if you’re around to hear it.
Cracks formed this way usually appear on the south and west sides of the trunk since these are the last places heated by the sun’s rays. A tremendous temperature drop can occur at sunset. If all sides of the trunk were heated and cooled evenly, tension and frost cracks wouldn’t develop because all tissues would expand and shrink at the same rate.
The lower temperatures fall, the greater the drying—creating conditions in which a tree is more likely to crack and in which the wider the cracks can become.
Apple and crabapple (Malus), ash (Fraxinus), beech (Fagus), Goldenraintree (Koelreuteria), horsechestnut (Aesculus), linden (Tilia), London plane and sycamore (Platanus), certain maples (Acer), tuliptree (Liriodendron), walnut (Juglans) and willow (Salix) suffer most.
Exposed trees are more susceptible than those in woodland areas, and trees at their most vigorous age (six to ten inches in diameter) are more susceptible than old ones. Probably because of the higher moisture content in their tissues, trees growing in poorly drained sites are more subject to cracking than those growing in drier, better-drained soils.
In spring sap rises, the wood absorbs water and the crack closes. The cracked zone in the heartwood never completely heals, even though the surface may be sealed by callous formation; and the same cracks tend to open again each winter. The repeated splitting and healing eventually results in a mass of callous over the crack.
Some trees eventually grow large enough in diameter or grow enough bark that cracking ceases. Until that happens, there is little that can be done. Don’t fill the crack. It will close by itself. Bolting frost cracks closed seldom works because the forces are so great that new splits will open along the bolts if split-inducing conditions recur.
When it is closed, trim away loose bark so that nothing can take up residence beneath it, and paint the wound with tree paint. It does not help the tree; but it looks better, and you will feel better.”
If you have other horticulture questions, call the Peoria County Extension Office. The number is 309-685-3140 or on the web at: www.extension.uiuc.edu/peoria.