How to Use Weeds to Read Soil

Article by Tom Oder

How to read a weed

Lawn weeds fall into two broad categories: broadleaf weeds and grassy weeds. Broadleaf weeds are generally the easiest to recognize because, as their name implies, they have a stem that often produces wide leaves frequently in pairs or groups. Exceptions are weeds such as dandelions, which have just a single leaf. In short, a broadleaf weed doesn’t resemble a grass, which is what can sometimes make grassy weeds a little difficult to recognize — at first glance, these weeds do look like grass.

Here are some of the most common broadleaf and grassy weeds that Waltz says are most likely to occur in home landscapes, as well as how to identify them and the problems they may indicate.

Prostrate spurge (Chamaesyce maculata and Euphorbia supina)

Prostrate spurge in your yard could be a sign that your soil needs to be aerated.  MaryAnne Campbell/Shutterstock

This is a vigorous, low-growing, broadleaf summer annual that forms a mat up to three feet in diameter. It’s often found in newly established or thin lawns. It gets its name from freely branching prostrate stems that usually have a reddish spot. It can indicate several possible problems with your soil. One is that the soil may be compacted and needs aeration. This plant, for example, will grow on cracks in sidewalks and parking lots. It may also indicate the presence of nematodes.

“If you have a high spurge population in your lawn, it’s worth at least taking a soil sample and sending in that soil sample to see if nematodes are really the problem with your lawn, not so much the weeds,” says Waltz. “It’s not foolproof, of course, but it is an indicator plant of nematodes.”

Annual bluegrass (Poa annua) and goosegrass (Eleusine indica)

Annual bluegrass is a cool season, grassy weed that is light green in color and grows in small tufts or clumps. Goosegrass, also called crowfoot and silver crabgrass, is a tough, clumped summer annual grass, generally with a “whitish to silverfish” coloration at the center of the plant. They are indicators of compacted soil. “Both of these do very well on shallow soils where the (desirable grass) roots can’t get down deep into the soil,” says Waltz.

Yellow woodsorrel (Oxalis stricta)

This is a broadleaf weed with three heart-shaped leaves that produces yellow flowers. Other forms of this plant that gardeners may encounter include creeping woodsorrel, (Oxalis corniculata), which has a more prostrate growth habit than yellow woodsorrel but may be green to reddish purple, and Florida yellow woodsorrel, which is similar in appearance to yellow woodsorrel. These weeds are sometimes an indication of low soil fertility, says Waltz.

Broom sedge (Andropogon virginicus)

This is a perennial grassy weed that sends up several tall stems from a basal crown. Its flowers are green to reddish-purple and will turn the color of straw when the seed heads mature. “If you see this, odds are good your soil is going to be a little low in pH,” says Waltz. To determine if this is the case, send a soil sample to your extension office. In pastures or hayfields with a lot of broom sedge, farmers are sometimes urged to put lime on their pastures because lime will raise the pH of the soil. “Many times, that will take care of the broom sedge because it doesn’t like the pH that pastures and forage grasses will grow in,” says Waltz.

Some weeds are hard to read

Some common weeds grow in so many environmental niches they don’t provide a clear indication of a soil condition.

One of those is the frequently seen dandelion (Taraxacum officinale). Dandelions are a broadleaf weed with a deep tap root, which Waltz found to his great surprise that some people view in a different light than he does. He encountered such a person at an extension talk on weed control he was giving as a graduate student at Clemson University.

“I thought I had not knocked it out of the park, and this guy raises his hand and says, ‘You know what weeds are in my lawn?’ I said, ‘No sir. What?’ He said. ‘They are a salad.'” While there are certainly weeds like dandelions that are edible, Waltz suggests caution if you see weeds as a chance to eat your yard. “If you pick the wrong thing, it can cause a little intestinal distress,” he points out.

Wild garlic (Allium vineale) and dog fennel (Eupatorium capillifolium) are two other common weeds that don’t indicate a particular soil type or condition. “I have seen them in clay and sandy soils,” says Waltz.

Sometimes the problem is you

Sometimes weeds set up shop in your yard because of poor lawn management practices.

“If I’m seeing certain weeds like dog fennel, American burnweed (Erechtites hieraciifolia) or marestail (Conyza Canadensis), that gives me an indication of poor maintenance,” says Waltz. “Some of those weeds like to grow up head high, five to seven feet tall. If you are seeing a lot of those, it gives me an indication the homeowner is not doing what they need to do to maintain the lawn at proper mowing heights.”

Because these weeds want to grow tall, even though they germinate and begin growing, they can’t survive in a lawn that is mowed frequently at the proper height. Regular maintenance just puts too much pressure on them, Waltz adds.

Are some grasses more susceptible to weeds than others?

Because of differences in their growth habits, some grasses are more likely to provide an environment that’s more conducive to weeds than other grasses. Lawns of tall fescue, which has a clumping and open growth habit, are more likely to have weed issues than lawns with thick-growing grasses such as zoysia grass, Bermuda grass, St. Augustine grass and centipede grass.

“Tall fescue is a cool-season species that can have a more open canopy following summer heat stress that promotes an environment for weeds,” says Waltz. “Likewise, it is more susceptible to disease. So, when it gets a pathogen or disease, it opens up the canopy so light and water get down to the soil and allow the weed seed to germinate and come up. Zoysia grass has a much denser canopy and excludes more light and, as a result, the grass will many times out-compete the weeds. Therefore, we tend to have fewer weed issues in zoysia grass than we do in other grass species.”

How to take a soil sample

Your local extension service is a good first contact to confirm what type of weed, disease or insect problem you might have in your lawn. They may suggest you email them a photograph of the weed or grass stems or suggest you send a soil sample to the extension’s lab at your state’s land grant university.

If they suggest a soil sample, here’s how Waltz suggests taking that sample based on an average-size lawn of 5,000-8,000 square feet: Pull 15-20 samples, 1/2 to 3/4 inches in diameter in the upper 3 to 4 inches of your soil, which is the root zone for most turf species. Take the turf canopy off the samples, mix the soil together, securely seal it in a plastic bag and take it to your local county office. They will then send it to your state’s extension lab.

Rules are made to be broken

Waltz acknowledges that there are no hard-and-fast rules about weeds as indicator plants; they’re more like guidelines. “When you see (weeds), it triggers another thing to consider about why the lawn may be less healthy than it should be and where you might want to tweak or address an issue.”

If you come across an oxalis in your lawn, for instance, he said you might want to take a soil sample, send it to the extension office and see if you need to make a nitrogen application. “Unfortunately,” he says, “on that one just increasing the fertility doesn’t always get rid of the weeds. That’s not a herbicidal strategy. It’s just an indicator that the weed is more competitive in that soil than the turf itself. This gets back to the definition of a weed — it’s competing for light, water, space and nutrients.”

Source: How to Use Weeds to Read Soil (treehugger.com)