Gardening Guides
What Your Weeds May Be Trying to Tell You
An invasion of weeds can reveal something about your soil. Here’s what is going on and what to do about it
Weeds are ubiquitous. They produce large volumes of seeds that find their way into every corner of the landscape, particularly places with disturbed soils. Where they find appropriate conditions they will germinate and grow. By knowing some things about a particular weed’s preferences and tolerances, we can make inferences about the conditions on the site where they are growing. While such a weed analysis won’t give specific details the way a soil test does, it can give you general information about the real-time growing conditions.
Plantain grows in a compacted area of a lawn, along with remnants of chickweed.
Compacted soil. This may be from foot traffic or heavy equipment. It is particularly bad if soil was wet when it was compacted. Weeds found here include plantain (Plantago spp.), dock, chicory (Cichorium spp.), dandelion and chickweed. Except for chickweed, these plants have very tough root systems that are able to penetrate through compacted soils.
Compacted soil. This may be from foot traffic or heavy equipment. It is particularly bad if soil was wet when it was compacted. Weeds found here include plantain (Plantago spp.), dock, chicory (Cichorium spp.), dandelion and chickweed. Except for chickweed, these plants have very tough root systems that are able to penetrate through compacted soils.
This is how yellow nutsedge often appears in the lawn. It is yellowish green, stiffer and more upright than normal lawn grasses. Left unmown, it will quickly grow taller than the surrounding grasses.
Wet or poorly drained soil. These are soils that are almost always wet, either from a constant influx of moisture or because of some impediment to drainage like clay, hardpan or bedrock. In consistently moist lawns, yellow nutsedge (Cyperus esculentus) is a very common weed. Other indicator weeds of wet sites include spotted ladysthumb (Polygonum persicaria), smartweed, ground ivy, violet, moss, chickweed, sorrel and other kinds of sedge.
10 Solutions for Soggy Soil
Wet or poorly drained soil. These are soils that are almost always wet, either from a constant influx of moisture or because of some impediment to drainage like clay, hardpan or bedrock. In consistently moist lawns, yellow nutsedge (Cyperus esculentus) is a very common weed. Other indicator weeds of wet sites include spotted ladysthumb (Polygonum persicaria), smartweed, ground ivy, violet, moss, chickweed, sorrel and other kinds of sedge.
10 Solutions for Soggy Soil
Common yellow oxalis is a common weed native to North America. It will concentrate in acidic soils as well as in soil that is fertile and high in clay.
Acidic soil. Soil pH is a measure of its acidity (sourness) or alkalinity (sweetness), with a pH of 7 being the neutral point. Plants can grow in a range of soil pHs. In general, shrubs and trees prefer more acidic soils, below 6.5, and grasses and perennials prefer neutral conditions, 6.5 to 7.5. When the soil pH reaches much below 5.5, the growth of lawns and many garden perennials is stunted. Weeds that do well in acidic soils are sorrel, plantain, spotted ladysthumb, common yellow oxalis (Oxalis stricta), moss and mullein (Verbascum spp.).
Have Acidic Soil in Your Yard? Learn to Love Gardening Anyway
Acidic soil. Soil pH is a measure of its acidity (sourness) or alkalinity (sweetness), with a pH of 7 being the neutral point. Plants can grow in a range of soil pHs. In general, shrubs and trees prefer more acidic soils, below 6.5, and grasses and perennials prefer neutral conditions, 6.5 to 7.5. When the soil pH reaches much below 5.5, the growth of lawns and many garden perennials is stunted. Weeds that do well in acidic soils are sorrel, plantain, spotted ladysthumb, common yellow oxalis (Oxalis stricta), moss and mullein (Verbascum spp.).
Have Acidic Soil in Your Yard? Learn to Love Gardening Anyway
Bladder campion is an attractive weed that is often found in more alkaline soils; photo by Andrew Coombes
Alkaline soil. A soil with a pH above 8.5 is considered strongly alkaline and unsuitable for many plant species, particularly trees and shrubs. These soils are more common in drier climates or where too much lime has been applied. Common weeds that can tolerate these soils are Virginia pepperweed (Lepidium virginicum), goosefoot (Chenopodium spp.), bladder campion (Silene latifolia), scarlet pimpernel (Anagallis arvensis) and burdock (Arctium spp.).
Grow a Beautiful Garden in Alkaline Soil
Alkaline soil. A soil with a pH above 8.5 is considered strongly alkaline and unsuitable for many plant species, particularly trees and shrubs. These soils are more common in drier climates or where too much lime has been applied. Common weeds that can tolerate these soils are Virginia pepperweed (Lepidium virginicum), goosefoot (Chenopodium spp.), bladder campion (Silene latifolia), scarlet pimpernel (Anagallis arvensis) and burdock (Arctium spp.).
Grow a Beautiful Garden in Alkaline Soil
Quackgrass spreads rapidly by underground runners, shown here. It is particularly successful in soils with hardpan.
Hardpan and crusty soil. Hardpan is a dense layer of soil that occurs a few inches to a couple of feet below the soil surface. It blocks both drainage of water and penetration by roots. It may be caused by the mineral composition of the soil or by mechanical compaction. Crusty surface soil is often associated with hardpan. It is the result of water acting on unprotected soils and breaking down the soil structure. When this finely dispersed soil dries out, it forms a hard, dense layer that cracks into plates, much like what you would see in the bottom of a dried-up pond. These conditions are also associated with salty soils.
Weedy plants that are commonly found in these soils include mustard, disc mayweed (Matricaria discoidea), bindweed (Convolvulus spp.), horsenettle (Solanum spp.) and quackgrass (Elytrigia repens). These last three perennial weeds spread by underground runners (rhizomes) through the soil layer between the hardpan and the hard upper crust of these soils.
Hardpan and crusty soil. Hardpan is a dense layer of soil that occurs a few inches to a couple of feet below the soil surface. It blocks both drainage of water and penetration by roots. It may be caused by the mineral composition of the soil or by mechanical compaction. Crusty surface soil is often associated with hardpan. It is the result of water acting on unprotected soils and breaking down the soil structure. When this finely dispersed soil dries out, it forms a hard, dense layer that cracks into plates, much like what you would see in the bottom of a dried-up pond. These conditions are also associated with salty soils.
Weedy plants that are commonly found in these soils include mustard, disc mayweed (Matricaria discoidea), bindweed (Convolvulus spp.), horsenettle (Solanum spp.) and quackgrass (Elytrigia repens). These last three perennial weeds spread by underground runners (rhizomes) through the soil layer between the hardpan and the hard upper crust of these soils.
This common sheep sorrel is growing through a bed of wood chips. It is particularly fond of dry or acid conditions.
Dry soil. Overly dry soils may be because of excessive drainage (sandy soil) or a lack of moisture due to drought or being in a rain shadow or covered area, such as under the eaves of a house. Some weeds that are adapted to such conditions are common sheep sorrel (Rumex acetosella), black medic (Medicago lupulina), prostrate pigweed (Amaranthus albus), common yarrow (Achillea millefolium), mullein, common gypsyweed (Veronica officinalis), rabbitfoot clover (Trifolium arvense) and sweetclover (Melilotus albus).
Gardening Solutions for Dry, Sandy Soils
Dry soil. Overly dry soils may be because of excessive drainage (sandy soil) or a lack of moisture due to drought or being in a rain shadow or covered area, such as under the eaves of a house. Some weeds that are adapted to such conditions are common sheep sorrel (Rumex acetosella), black medic (Medicago lupulina), prostrate pigweed (Amaranthus albus), common yarrow (Achillea millefolium), mullein, common gypsyweed (Veronica officinalis), rabbitfoot clover (Trifolium arvense) and sweetclover (Melilotus albus).
Gardening Solutions for Dry, Sandy Soils
Here we see a mixture of three legumes — crown vetch (with lavender flowers), white clover and black medic (with yellow flowers) — growing together. This combination indicates low soil fertility.
Poor soil. Soils that are of low fertility due to nutrient leaching or overuse are low in one or more of the nutrients needed for plant growth, particularly nitrogen, phosphorus and potassium. Clover (Trifolium spp.) and other legumes are common in nitrogen-poor soils since they are able to convert nitrogen from the air into a form usable by plants, called nitrogen fixation. Other weedy plants that show up in low-fertility soils are common sheep sorrel, plantain, common mullein (Verbascum thapsus) and mugwort. In forests, western brackenfern (Pteridium aquilinum) is an indicator of low fertility.
Poor soil. Soils that are of low fertility due to nutrient leaching or overuse are low in one or more of the nutrients needed for plant growth, particularly nitrogen, phosphorus and potassium. Clover (Trifolium spp.) and other legumes are common in nitrogen-poor soils since they are able to convert nitrogen from the air into a form usable by plants, called nitrogen fixation. Other weedy plants that show up in low-fertility soils are common sheep sorrel, plantain, common mullein (Verbascum thapsus) and mugwort. In forests, western brackenfern (Pteridium aquilinum) is an indicator of low fertility.
Oxalis and goosefoot grow vigorously in fertile soils. Goosefoot is also known as lamb’s quarters and wild spinach.
Good, fertile soil. Besides indicating soil problems, weeds can also indicate that the soil has particularly good fertility. These include chickweed, chicory, henbit, goosefoot, pigweed (Amaranthus spp.), common yellow oxalis and burdock. An area where you find many of these plants growing may be a good site for a new vegetable garden.
What to do? Armed with this knowledge about your soil conditions, you can do several things. You can change the conditions to make them less favorable to the weeds. This includes soil aeration to break up compaction, changing how water is distributed — for wet or dry soils — or changing how you fertilize, which will affect soil fertility and pH.
Before making any major chemical adjustments to your soil, do a soil test to confirm the conditions and get an idea of the amounts of treatments that are needed. You can also work with the soil type you already have and plant other species that are better adapted to the current conditions.
More
Get the Dirt on Your Garden’s Soil
How to Design Your Landscape to Sink Water Into the Ground
10 Tips for Beginning Gardeners
Zen Weeding: The Smart Way to Weed Your Garden
Good, fertile soil. Besides indicating soil problems, weeds can also indicate that the soil has particularly good fertility. These include chickweed, chicory, henbit, goosefoot, pigweed (Amaranthus spp.), common yellow oxalis and burdock. An area where you find many of these plants growing may be a good site for a new vegetable garden.
What to do? Armed with this knowledge about your soil conditions, you can do several things. You can change the conditions to make them less favorable to the weeds. This includes soil aeration to break up compaction, changing how water is distributed — for wet or dry soils — or changing how you fertilize, which will affect soil fertility and pH.
Before making any major chemical adjustments to your soil, do a soil test to confirm the conditions and get an idea of the amounts of treatments that are needed. You can also work with the soil type you already have and plant other species that are better adapted to the current conditions.
More
Get the Dirt on Your Garden’s Soil
How to Design Your Landscape to Sink Water Into the Ground
10 Tips for Beginning Gardeners
Zen Weeding: The Smart Way to Weed Your Garden
When using weeds or other plants as indicators of site conditions, don’t rely on a single species, or one or two plants. Look for multiple species that provide a consistent message. Perennial weeds generally are better indicators of site conditions than annuals. Read on to learn about common landscape situations and the weeds that frequent them.
Learn more about each type of weed you might see in your garden