Experts tackle readers’ garden questions.
Q: I planted my saucer magnolia about a dozen years ago when it was a seedling in a 2-gallon container in a tiny 11- by 15-foot rectangle of “lawn” between my garage and my neighbor’s garage in East Sacramento. The only thing growing on it was grass, which continues to grow, although it’s getting sparse under the tree as the tree gets bigger. The tree must be happy because it is now about 20 feet tall and wider than the plot. I hand water the tree, usually once per week. In cold weather, I do not water the tree at all.
I love the tree, but want to save as much water as possible, so am wondering if I should get rid of the grass? If so, is sheet mulching the best method to avoid harming the tree? What should I use to replace the grass after it’s gone?
Maris Montanet, Sacramento
Sacramento County Master Gardener Cathryn Rakich: Saucer magnolias (Magnolia x soulangeana) can be seen throughout the Sacramento area, exploding with large, show-stopping pink, purple and white blossoms on bare silver-gray branches in early spring. Those blooms are followed by a canopy of large dark green leaves in summer and fall. These deciduous beauties can reach as high as 20 to 30 feet and just as wide. They do best in rich, acidic, well-drained soil with full sun to part shade.
Magnolias are most often found planted within a lawn; however, as with most trees, they fare better with a good layer of organic mulch. Consider trees in a forest. The forest floor is covered with twigs, leaves and rotting wood, not grass. This natural mulch helps moderate the temperature around the tree roots, adds organic matter and nutrients to the soil, and helps retain moisture. Saucer magnolias, in particular, do not tolerate soil that is too dry or too wet, and do best with consistent irrigation. A good layer of mulch will ensure the conditions remain moist with adequate oxygen to the soil. Turf, on the other hand, competes with the magnolia’s roots, which remain close to the surface, for nutrients and water.
In addition, many gardening practices intended to keep lawns looking good – such as chemicals to kill weeds, heavy fertilizers and mowing – are not beneficial for trees. Saucer magnolias have a thin bark that is easily damaged by lawnmowers and weed cutters, which can lead to wounds and rot. A good organic mulch also suppresses weeds, and reduces compaction and erosion from irrigation, rainfall and foot traffic.
The good news is that the grass does not need to be removed before mulching. Sheet mulching with cardboard is an inexpensive way to remove a lawn that does not require chemical application or sheets of plastic.
Follow these four easy steps: Cut the lawn as low as possible; cover it with a single layer of cardboard (overlap the edges to prevent weeds and grasses from coming through); dampen the cardboard; and add 4 to 6 inches of organic mulch. The cardboard and mulch cut off sunlight and air to the grass and provide a barrier to its growth. Eventually, the grass will die and the cardboard will break down, adding carbon back into the soil.
Mulch is any material used to cover and protect the soil. Many types of landscape mulches are available, including organic mulches that can improve soil conditions as they decompose. They include shredded bark, wood chips, compost, grass clippings and leaves. Organic mulch will need to be periodically replenished as it decomposes, moves and settles.
Synthetic mulches such as black plastic will restrict oxygen and water to the soil and should not be used with trees and other plants. However, manufactured synthetic mulches called geotextiles or landscape fabrics allow water and air to pass through. Inorganic mulches such as lava rock, gravel and crushed stones are not recommended as they can lead to soil compaction and do not add beneficial organic matter.
Remember to keep the mulch at least 6 inches from the tree trunk to avoid promoting root and crown diseases. The fallen leaves of this deciduous tree can remain on the ground adding to the mulch.
For more information on mulching, visit the University of California Integrated Pest Management website at ipm.ucanr.edu.
Cathryn Rakich is a UC Cooperative Extension Master Gardener for Sacramento County.
Questions are answered by master gardeners at the UC Cooperative Extension services in Sacramento and Placer counties. Send questions to Garden Detective, P.O. Box 15779, Sacramento, CA 95852. Send email to email@example.com. Please put “Garden Detective” in the subject field and include your postal address. To contact UC Extension directly, call:
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