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Weed Control Methods for Oak Plantings

Oaks ‘n’ Folks – Volume 13, Issue 1 – February, 1998

One of the main obstacles to establishing oaks artificially is weed competition. Much of the hardwood rangeland in California has a dense understory of introduced Mediterranean annuals. Unfortunately, these annuals are vigorous competitors for moisture and nutrients-especially in early spring-and can make conditions very difficult for oak seedlings to become established. Researchers at the Sierra Foothill Research and Extension Center (SFREC) have conducted several experiments on weed control around oaks and have determined that controlling weeds in oak plantings is critical to successful regeneration. Eliminating competing plants around young trees not only frees up valuable resources necessary for survival and growth, but also removes a habitat that is particularly favorable for grasshoppers and voles, two animals that can severely damage oak seedlings by defoliating them and/or stripping bark from their main stems. Where this vegetation is removed or eliminated from around seedlings,damage is greatly reduced or eliminated entirely.

For these reasons, always try to provide some weed control around new plantings of oaks-at least for the first two years after establishment. Weed control can be accomplished in a variety of ways: physical removal(scalping, mowing, or weed-eating); weed suppression (using chips, compost,straw); physical barriers (black plastic or semi-permeable mats) that block sunlight but permit water to percolate through; or chemical application.

Several years ago researchers at SFREC initiated an experiment to compare four weed-control treatments around young oak seedlings, including weed-free circles with diameters of 2, 4, and 6 feet, and a control with no weed removal. All treatments resulted in significantly better survival, height growth,and diameter growth than the control. Larger weed-free circles increased seedling growth the most. However, of the treatments that were evaluated,scalping a 4-foot circle was best, when the benefits of the treatment were weighed against the difficulty of providing it.

Weeds also have been eliminated around oaks using lawn mowers and weed-eaters. These treatments have their limitations because they remove only the tops of the plants and do not kill them. The plants will grow back rapidly if mowing is conducted early in the growing season. Therefore, mowing only in early- or mid-summer, when most annuals have stopped growing and are turning brown generally is recommended. Although the senesced annuals do not compete seriously with oak seedlings, they still provide ideal habitat for voles and grasshoppers. Therefore, cutting them back reduces the potential for future animal damage.

A variety of organic and inorganic materials can be used as mulches around young oaks. All of these materials tend to suppress weeds by physically covering them, eliminating the light necessary for photosynthesis and growth. They also tend to conserve soil moisture by reducing evaporation from the soil surface, resulting in more available moisture for the oak seedlings. problem with mulches is that they do not last forever. Plastics tend to become brittle and degrade in sunlight, while organic materials gradually decompose. Occasionally, weeds will also grow up through holes in the plastic,or through shallow places in the organic mulch. For maximum benefit, these weeds should be regularly removed.

When using chemicals to eliminate weeds around oaks, using the proper herbicides and following the label on the container is critical, so that only the competing target plants are affected, and the oaks are not damaged.n most of the oak studies conducted at SFREC, researchers have used Roundup(glyphosate) to eliminate weeds. This is a broad-spectrum, post-emergence herbicide that affects a wide range of plants. Spray it around planting spots in the early spring, after most of the weed seeds have germinated.f the oaks have leafed out, or shoots from newly germinating acorns have emerged from the soil, not spraying them is very important, because they can be seriously damaged by the herbicide. At SFREC, glyphosate has been sprayed directly over the tops of dormant deciduous oaks (with no leaves)in the late winter, but a few usually sustained some damage if their buds were swollen. Therefore, when applying Roundup around oaks, the seedlings generally should be covered or protected, and chemicals should be directionally sprayed away from them.

A problem with Roundup is that if the chemical is applied in the early spring (which is generally necessary to get rid of the weeds before they deplete the soil moisture), it does not kill all of the competing plants such as starthistle, mustard, or turkey mullein, which germinate late in the season. Consequently, in late spring and early summer, these plants often create a new batch of competitive weeds that need to be removed by one of the methods already described. Finally, herbicides generally are the easiest and cheapest method of weed control, but require great care in their application.

A variety of approaches can be used to successfully address the problem of weed competition around planted oak seedlings. Though I strongly recommend that some type of weed control be incorporated into an oak planting program-especially during the first two years-the actual procedure or technique chosen may depend on many variables, including soil, topography, equipment or materials available, weeds present, oak species planted (deciduous or evergreen),and even one’s attitude about the use of herbicides. Whichever method is chosen, however, weed control will greatly improve the chances for the success of oak plantings by both reducing competition and eliminating habitat for damaging animals.

prepared and edited by Richard B. Standiford and Pamela Tinnin