Radial Trenchıng

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

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


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