Here are the most reliable weedy indicators and what they reveal about your soil.
Photo: (cc) Howard Dickins/Flickr
If you see dock, foxtails, 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.
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.
Photo: (cc) Eugene Zelenko/Flickr
Chicory 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.
Photo: (cc) Joost J. Bakker/Flickr
Dandelions, mullein, sorrel, stinging nettle, 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.
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.
Photo: (cc) Bob Jenkins/Flickr
Campion, field peppergrass, nodding thistle, 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.
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.
Biennial wormwood, common mullein, daisies, 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.
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.
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Fertile soil will often make its richness known by supporting happy and vigorous colonies of chickweed, 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.
Keep Reading: 8 Weeds You Can Eat.
Photo: (cc) Steve Ryan/Flickr