What The Weeds In Your Yard Can Tell You About Your Soil Quality

You can figure out what plants will thrive in your area just by looking at your local weeds.

May 16, 2017
dandelion
Daniel Viñé Garcia/ Getty

What do you do when you see a weed in the garden? Jump in and frantically hack away with a hoe? Throw up your hands in despair? Learn something?

Yes, learn something! Those weeds are excellent indicators of soil conditions. In fact, experts known as geochemical botanists often look for specific weeds to help them locate minerals in the soil and to pinpoint geological features. You can apply this science in your own backyard in two ways: to plant garden crops that will thrive in the same conditions as those weeds or to amend your soil so that the conditions are less inviting to the weeds you find there. (Check out these 7 common backyard weeds that are actually medicinal herbs in disguise.)

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Here are the most reliable weedy indicators and what they reveal about your soil.

(Whether you're starting your first garden or switching to organic, Rodale’s Basic Organic Gardening has all the answers and advice you need—get your copy today!)

Weeds That Say Your Soil Is Soggy

foxtail weed
Vinod Kumar / EyeEm/ Getty

If you see dock, foxtails (shown above), horsetail, and willows, you can expect your site to suffer swampy conditions some time during the year. Other weeds that thrive in wet soil include goldenrod, Joe-Pye weed, oxeye daisy, poison hemlock, rushes, and sedges.

Related: 8 Weeds You Can Actually Eat

What could you possibly grow in such conditions? How about a fabulous garden filled with plants that like wet feet? Ornamental willows, including pussy willow and curly willow, will flourish here and provide plenty of material for flower arrangements. You can also grow dogwoods, Japanese iris, Siberian iris, yellow flag, ligularia, cardinal flower, and turtlehead. Don't grow invasive wet-loving plants like purple loosestrife or meadowsweet, however. They can overwhelm the area and destroy the natural balance of the wetlands.

Also, don't try to change these conditions. Wetlands are priceless natural habitats that are rapidly being lost to development. Besides, trying to "correct" such a site usually is a lost cause—in Nature, water almost always wins.

Weeds That Cry Out "Compaction and Crust"

chicory weed
kellyvandellen/ Getty

Chicory (shown above) and bindweed are telltale signs of compacted soil. That's why you often see the blue flowers of chicory along highways. Chicory also is common in gardens where beds have been left empty or, worse still, where soil has been worked when it's wet.

If your weeds indicate compacted soil, plant a cover crop of white lupines and sweet clover. They have roots as strong as those of pesky chicory, and they help to break up the soil. At the same time, these cover crops replenish the nitrogen levels in the soil.

Although a hard crust on your soil's surface can prevent many vegetables and flowers from breaking through, it doesn't deter quackgrass or mustard family weeds at all. If weedy mustard is flourishing in your garden, pull it up and plant closely related brassica crops such as broccoli, cabbage, cauliflower, and choy instead. They can push through crusty soil with ease. Replace quackgrass with a fast-growing grassy cover crop (such as rye) in fall, then till it under the following spring. The cover crop will loosen the soil and choke out the weeds.

To aerate and lighten crusty and compacted soil, add compost. Prevent future problems by working your soil only when it's dry.

Weeds That Signal Your Soil Is Sour

stinging nettle
Tom Shower / EyeEm/ Getty

Dandelions, mullein, sorrel, stinging nettle (shown above), and wild pansy all thrive in "sour" acidic soil (pH below 7.0).

If you find these pests in your garden, grow plants that also like their soil on the tart side: hydrangeas (whose flowers achieve their most beautiful shades of blue in acidic soil), blueberries, rhododendrons, and azaleas. In the vegetable garden, endive, rhubarb, shallots, potatoes, and watermelon all tolerate soil with a pH as low as 5.0.

Related: The Secret To Perfect Organic Garden Soil

Or, if you'd rather grow plants that thrive in neutral soils, you could raise your soil's pH by applying dolomitic limestone. To determine how much lime to use, send a soil sample to a lab for testing, then follow the lab's recommendations. Wood ashes also will raise soil pH, but don't use any more than 25 pounds per 1,000 square feet, and avoid applying them more often than every 2 or 3 years. Compost is a better buffer: Just add enough to raise your soil's pH to 6.5 or 6.8.

Weeds That Say Your Soil Is Sweet

nodding thistle
RuudMorijn/ Getty

Campion, field peppergrass, nodding thistle (shown above), salad burnet, scarlet pimpernel, and stinkweed all indicate a "sweet" alkaline soil (pH higher than 7.0).

Ornamentals that do well in alkaline soil include lilacs, Persian candytuft, dianthus, baby's breath, helianthemum, dame's rocket, lavender, and mountain pinks. Some edibles also tolerate soil that's a little on the sweet side, including asparagus, broccoli, beets, muskmelons, lettuce, onions, and spinach.

Related: Our Top 10 Tools For Working The Soil

If you want to lower the pH of your alkaline soil, add peat moss or elemental sulfur at a rate suggested by soil test results. Or, again, simply add compost regularly to bring the pH closer to neutral.

Weeds That Warn of Worn-Out Soil

daisies
Ron Bambridge/ Getty

Biennial wormwood, common mullein, daisies (shown above), mugwort, wild carrot, wild parsnip, and wild radish are sure signs that your soil has poor fertility.

Luckily, many perennials actually flower better when the amount of food in the soil is on the lean side. This list includes achillea, antennaria, artemisia, asclepias, centranthus, cerastium, coreopsis, echinops, eryngium, gaillardia, salvia, santolina, solidago, and stachys. In the edible department, beans (and other legumes), beets, carrots, parsnips, peas, radishes, sage, and thyme tolerate soil that's low in fertility.

Related: How Crop Rotation Can Help You Manage Pests And Improve Soil Quality

Of course, you could and should improve the fertility of at least some of that soil. First, have your soil tested. If the test reveals major deficiencies, correct them with organic fertilizers such as fish meal (for nitrogen), bonemeal (for phosphorus), and greensand (for potassium). From then on, use compost and cover crops to maintain your soil's fertility.

Weeds That Reveal Your Soil Is Rich

chickweed
Petar Dyulgerov/ Getty

Fertile soil will often make its richness known by supporting happy and vigorous colonies of chickweed (shown above), henbit, and lamb's-quarter. In addition, a flush of redroot pigweed indicates an abundance of nitrogen in the soil, while knapweed and red clover reveal an excess of potassium. Spot lots of purslane and mustard? They could be telling you that your soil is rich in phosphorus.

To take full advantage of your soil's fertility, plant heavy feeders, such as corn, broccoli, lettuce, melons, squash, tomatoes, and peppers.

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