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The Garden Plot for March 21, 2008
3:55 PM - 4:00 PM
[Program Website]
Today's Highlight: “Planting Trees and Shrubs”
It’s time to start thinking about planting trees and shrubs. You can plant trees and shrubs almost any time of the year, but the best time to plant is when they are dormant, before buds have broken and green leaves emerge. As soon as the ground can be dug into, dig a hole deeper and wider than the root system of the plant you plan to transplant. If you have rich, dark, fluffy soil that forms a loose ball when you moisten it, and then crumbles easily, you may not need to worry about how big the planting hole is, or whether or not to add a soil amendment.
Unfortunately, most of us in western Montana don’t have the perfect soil for the trees and shrubs we want to plant. If your soil is a heavy clay (it forms a tight ball when moistened, and sticks to your hand) or if your soil is a light sand (it won’t form a ball at all, even when quite moist), then you might want to consider digging a bigger hole and using a technique called “radial trenching”. Radial trenching means that you did a hole as deep and wide as the root system of your transplant, then you dig 3-6 horizontal trenches out from the main hole, in a star shape about 6 inches wide and 10 to 16 inches deep. These radial trenches allow roots to grow outside of the main planting hole, even if your soil is not particularly conducive to good root growth. In general, the roots of new transplants want to grow horizontally rather than vertically at first. Horizontal trenches give roots a quick start and help to overcome a condition called “transplant shock” in which trees and shrubs don’t develop a new root system fast enough to provide the water and nutrients they need the first year or two after planting.
Whether or not to add soil amendments to the transplant hole is a more complex question and depends on tree and soil type. In our Missoula County Extension soil amendment studies, we found that trees with the greatest first year shoot growth were growing in soil amended with either a compost made of fir bark plus biosolids or with no amendment added. The trees in our study with the most dieback were growing in soil amended with a 16-16-16 fertilizer treatment. However, new shoot growth on trees growing in the 16-16-16 treatment was almost as good as in the compost and no amendment treatments. The least dieback was found on trees growing in treatments amended with composted fir bark plus alfalfa meal and with compost.
In summary, our first year results don’t point to a clear winner, but suggest that when considering shoot growth and dieback right after transplanting, a lower pH, slow release plant nutrient compost addition may get some tree species off to a quicker start, without encouraging dieback.
Whether or not you add a soil amendment, make sure you add at least 5 gallons of water to the planting hole as you plant. First, return one half of the soil you dug out back to the hole and pour in 2-3 gallons of water. Then, add the remaining soil and water-in with another 2-3 gallons of water. This helps to ensure good initial soil-root contact.
For more information on tree planting you can contact your local county extension agent.
Helen Atthowe's new short program of gardening tips
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