There’s much to like about garden asparagus. It’s our premier perennial vegetable, winter-hardy and long-lived, yielding spears from early spring until harvest stops July 4. An asparagus patch can be located at one end of the vegetable garden or even located in a perennial flower garden.
There’s little fuss with asparagus. There’s no need to dig and divide for decades, once planted. Fern-like tops are best left on during fall/winter and are easily removed in early spring.
Little fuss, that is, until weeds encroach. 
Weed control is the greatest challenge in growing asparagus in our gardens. An asparagus planting remains in place for decades, undisturbed by tilling or cultivation, and so weeds can easily become established among asparagus plants. Perennial weeds, like quackgrass, dandelions and thistles easily entrench themselves in an asparagus patch, emerging every year from their own long-lived roots, just as the asparagus does. Their competition can easily diminish asparagus yield and vigor.
“How can I control weeds in my asparagus?” The question is short; the answer is not. Weed control in asparagus requires a combination of techniques.
How to Control Weeds in Asparagus: the multi-pronged approach.
- Hand weeding. Hoeing, cultivating and pulling weeds is sometimes the most effective method, especially important when the asparagus patch is new, to prevent perennial weeds from becoming entrenched.
- Mulch. 4-6 inches of straw, shredded bark, or herbicide-free grass clippings can reduce weed competition.
- Pre-emergent herbicides. (applied before weeds emerge.) Products like Preen, or Preen organic (containing corn gluten meal) are applied to clean, weed-free soil and prevent weed seed from establishing. It won’t control weeds that are already growing, or weeds that arise from perennial roots, like quackgrass, thistle, or dandelion. Pre-emergent herbicides can help reduce “annual” type weeds from sprouting in the asparagus.
- Post-emergent herbicides (applied after weeds emerge.) Consider using glyphosate (Original Roundup) or a grass-killing herbicide.
- Glyphosate. (The product in original Roundup and other brands.) Can be useful for treating thistles, quackgrass and others. Two easiest times to apply: (1) in early spring, before asparagus spears emerge from the ground, spray any weeds that are actively growing. (2) Late June/early July – during the last harvest, cut asparagus spears one inch below the soil surface, so no stems are visible, then spray the weeds that are growing in the patch. The asparagus will be safely below ground at the time of spraying, and will emerge after. Always read and follow label. If you have any hesitation about using glyphosate, consider other options.
Glyphosate (Original Roundup) can be applied to actively growing weeds after asparagus spears have all been cut one-inch below soil surface. - Grass-killing herbicides. Some herbicides have active ingredients that kill only grass-type weeds, such as quackgrass, so can be applied directly onto asparagus plants that are actively growing, without harming the asparagus. More than one application is usually required. Several brand names are Bonide Grass Beater and Hi-Yield Grass Killer. The waiting period between application and harvest is one day, according to the product labels.
- Do not use salt for weed control, which is a dangerous recommendation sometimes seen on the internet. Salt is very damaging to soil.
- Smothering: cardboard or newspaper can be laid on the soil right up to the asparagus plants. Cover with straw, shredded bark, or untreated dried grass clippings. The cardboard will smother weeds and will decompose into organic material, as it is left in place. You’ll still need to remove weeds within the asparagus plants themselves.
Many of us are aware of asparagus growing in ditches or along old shelterbelts, where it seems to coincide and grow among grass and other vegetation. But for a home garden, letting the patch become a wild blend of asparagus, grass, and weeds isn’t a wise recipe. Production, quality, vigor and yield are tops when asparagus is free of competition. 
Happy Gardening! 
This article is based on recommendations from North Dakota State University, University of Minnesota, Michigan State University, and University of Nebraska.
