How to Combat Soil Compaction Soil compaction is the number-one enemy of plant growth. Excessive foot or vehicular traffic is the most obvious culprit, but it can occur in time anyway. Saturated soils such as we have experienced this spring only exacerbate the condition. Most shallow-rooted plants such as perennials and small shrubs will eventually recover with regular top dressings of a good organic mulch, but large trees may not be so lucky. There is another method of applying organic material that may help for those worst-case scenarios.
Just as the name implies, radial trenching is a method that employs a series of trenches that radiate outward from the trunk of the tree. These shallow trenches are then filled with a mixture of soil and compost. New tree roots have been shown to grow into the trenched areas at a rate four times that of surrounding soil within just two seasons. For even greater effect, mycorrhizae (fungi that grow in symbiosis with tree roots) can also be added to the amended soil. Healthy roots with a complement of mycorrhizae are capable of up to seven times the absorption of water and nutrients as those without their symbionts.
There are some rules to follow in digging radial trenches. The trench should be narrow — as narrow as 4 inches to 24 inches in width — and 12-18 inches deep. Begin the trench at (or beyond) the drip line of the tree, digging inward toward the trunk. If you encounter roots in excess of 1 inch in diameter, carefully dig around them without severing or damaging them. End each trench no closer than 6 inches from the trunk for each inch of trunk diameter. For a trunk diameter of 24 inches, the trench would end 12 feet away, for example. After refilling with the amended soil, apply a 4-6 inch layer of woody mulch (wood chips or recycled green waste).
It may take several years to see the effects of this treatment in the top growth of the tree, so don’t be impatient. Your tree will be breathing easier soon. May Gardening Tips Subtropical fruits (citrus, avocado, guava) can be planted and pruned now. Also cut back bougainvillea, tibouchina, and abutilon. Plant a block of corn (i.e., four rows of four plants) for best pollination and full ears. Also plant peppers, tomatoes (watch out for hornworms), and eggplant. Try some colorful beans such as rattlesnake or royal burgundy (visit heirloomseeds.com/beans.htm). Wash off aphids, but leave a few so that beneficial insects will have a reason to come to your aid.
Virginia Hayes, curator of Ganna Walska Lotusland, will answer your gardening questions. Address them to Gardens, The Independent, 122 W. Figueroa St., S.B., CA 93101. Send email to email@example.com.