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Fall Pruning and Fertilization and Winter Protection
October 30, 2006

Although the weather has been lovely for vacations at the cottage, it’s been terrible for applications of fertilizer, root pruning and transplanting. Many landscape service providers have had to hold off on these routine practices because of a water deficit in the soil. A general rule of thumb is to get transplants in the ground by mid-October, so they have time to establish a root system before the soil temperatures dip down and the ground freezes. That’s all well and good but soil moisture has just been returned to a decent level last week. This means that everyone is now scrambling around to make those fall applications and plant installations. Take heart that many conifer roots can grow to a soil temperature down to –5 oC. Hopefully there is still enough time for root regeneration and water uptake before the ground freezes. A low to moderate application of nitrogen fertilizer on woody plants will help with nutrient absorption this time of year. The effects will show up next spring with the spring flush of growth. Soil moisture also plays a strong role in acclimation for winter hardiness. Those specimens receiving supplemental irrigation in early autumn should show a marked improvement in hardiness this winter.

Winter Protection

A lot of people think that winter kill is due to freezing temperatures. Well that’s really only the half of it. Remember all of the winter burn we saw on the southwest side of evergreens back in 2002/03? A lot of this type of damage is caused when there is ample snow cover around. The sunlight is reflected off the snow and onto the foliage of evergreens. The foliage warms up (ice crystals melt into water) and the foliage starts to transpire (conduct water). Then at night, the air temperature goes down past freezing, the water is frozen into ice crystals which then expand and cause the cells to burst. The process is repeated over several days, resulting in severely damaged tissue. Now remember, the roots are still frozen and during the day, the foliage is repeatedly asking for water to continue transpiration. The foliage soon runs out of water. Once you add those prevailing winter winds to the picture, the stressed foliage doesn’t stand a chance. Evergreen foliage dies and turns a rusty-brown. A similar phenomenon happens on the southwest side of thin-barked trees (Acer, Magnolia, Malus, Pyrus, Sorbus). Southwest injury on bark often results in vertical cracks. I’m sure you have seen lots of these.

So what can you do to prevent this?

Provide a barrier that restricts the sun’s heat and drying winds from getting at the bark and foliage. Some landscape service providers will wrap tree trunks (usually those less than 15 cm DBH, as a general rule) with burlap or some other cloth that offers some protection. It’s very important to remove the burlap in late winter/early spring so rot organisms don’t find a permanent home under the cloth. Others will paint the trunk with latex paint, but this may be unsuitable for some property owners. Evergreen shrubs and young trees can be protected by constructing a vertical frame (e.g. four stakes in the ground) and wrapping the frame with burlap. Avoid placing the burlap right next to the foliage. Burlap can become saturated with road salt and will actually cause burning if it is in contact with foliage. Burlap can also hold heavy, wet snow on the branches and cause structural damage. Most winter winds prevail from the northwest, whereas summer winds and the warming affects from the sun prevail from the southwest.

Jennifer Llewellyn, OMAF Nursery Crops Specialist with the
Ontario Ministry of Agriculture & Food
Dept. of Horticulture Science, University of Guelph

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