Producing fresh green shoots every spring for 10 years or more, asparagus (Asparagus officinalis officinalis) is a salt-tolerant perennial vegetable. Farmers used to spray salt solution around asparagus to control weeds, but nowadays this practice is discouraged due to its harmful environmental effects. Asparagus grows in U.S. Department of Agriculture plant hardiness zones 4 through 9 and plants die back each fall, leaving no living vegetation above ground. Cultural methods and herbicides provide effective weed control around asparagus plants.
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Controlling Weeds With Salt
Salt solution provides some weed control around asparagus plants, but its harmful effects outweigh its advantages. Spraying weeds in an asparagus patch with a salt solution controls only some weeds, and the salt forms a crust on the soil surface, slowing the penetration of water and damaging the soil structure. Salt also builds up in the soil over time, causing asparagus to grow poorly, and contaminating the ground, so that it's unfit for growing other crops. Clay soil is worst affected because it retains water. Sandy soil allows more salt solution to drain through, but it can then contaminate groundwater.
Alternatives to Salt
Gardeners who want to avoid spraying with herbicides can use cultural weed-control methods as an alternative to salt. Asparagus crowns, which are the shoot-producing areas of the plant, lie 4 to 8 inches below the soil surface. Hoeing the growing area shallowly in early spring before new asparagus shoots appear avoids damaging asparagus crowns and controls weeds. Spread a 3- to 4-inch layer of garden compost, wood chips, clean straw or other organic mulch to shade out new weed growth, and check the asparagus bed every week throughout the growing season, pulling out new weeds that appear. Top up the mulch with fresh organic mulch as it thins out.
Pre-emergent herbicides prevent weed seeds germinating around asparagus plants. Pre-emergent herbicides create a chemical barrier on the surface of the soil and prevent new weeds growing, but don't control established perennial weeds. Apply a pre-emergent herbicide in early spring before asparagus shoots appear, spreading a 1.47 percent trifluralin product at a rate of 1 ounce per 10 square feet on an established asparagus bed, or apply according to the manufacturer's instructions. Water the product after application to activate the chemical barrier, and don't disturb the soil, because this reduces the herbicide's effectiveness. A 1.47 percent trifluralin product provides up to three month's protection from germinating weeds.
Another alternative to salt as a weed control for asparagus includes post-emergent herbicides. Glyphosate controls deep-rooted perennial weeds, but also affects desired plants, so is only safe to use when asparagus plants have no living growth above ground. Put on a long-sleeved shirt, long pants and gloves, and spray weeds with a ready-to-use 2 percent glyphosate product on a dry, still day before asparagus spears appear in spring, and after cutting down the dead asparagus foliage in fall. Wash off any accidental splashes of glyphosate herbicide on desired plants immediately with plenty of water.