WHAT WEEDS CAN TELL YOU
Dr. Leonard P. Perry, Extension Professor
University of Vermont
Of course weeds can cause blisters on hands and sore backs from too much weeding, but they can also teach you about your soil and provide useful benefits. So give your hands and back a break. Look to see what weeds you have and which might be useful, as covered in the book by Sally Roth, Weeds, Friend or Foe?
Just as most ornamental plants have a preferred habitat, including soil conditions, so do weeds. As a result, spreading as they do, they tend to grow in their preferred conditions. So by looking at what weeds are growing, you can often tell what kind of soil conditions you have. One weed may grow in several soil conditions, but look at several growing together, and you have a pretty good clue what you have.
Knowing your soils better will help you decide if you need to change them, such as with drainage in wet soils, or peat moss to amend compacted soils. Knowing the soil will also help you better place ornamental plants for a more low maintenance and sustainable landscape.
If you have yellow nutsedge, dock, beggarticks, or heal-all, you probably have wet soil. Thistles, mullein, Bermudagrass (in the south), and purslane indicate dry soil. These soil types are often easy to tell without weeds, but what about fertility?
Shepherd's purse, mullein, and black medic indicate lean soil. Henbit, dead nettle, dandelion, and clover indicate fertile soil. If your soil is acidic, you may see sorrel, crabgrass, annual bluegrass, and plantain. If neutral, chickweed, dandelion, and wild mustard may be found. If alkaline (more seen in western gardens or specific locales), poppy and sagebrush are often found.
You can obviously tell if you have shade, in which case you should look for garlic mustard, ground ivy, nightshade, or poison ivy. Annual bluegrass, knotweed, and mouse-ear chickweed indicate you may have compacted soil. Loosen such soil by incorporating organic matter such as compost or peat moss, double digging deeply if in the garden, or aerating if under a lawn.
Weeds don't just tell you what soil you have by their presence, often they can improve your soil, which may seem an odd concept. Tap-rooted weeds such as dock, burdock, mallows, and dandelion push deeply through the soil. This opens and increases air spaces, basically aerating deeply, and so improves drainage even in clay soils. Pulling such weeds will then provide channels for water, air, and nutrients to deeply enter the soil.
Low and shallow-rooted weeds such as ground ivy, some knotweeds, and purslane act as groundcovers. These help prevent erosion, as well as soil crusting from hot and dry conditions. Soils that aren't crusted or baked hard will allow water and air to enter more easily.
Some legume family weeds such as clover and black medic have nitrogen sacs or nodules on their roots, thus adding nitrogen to soils and for your other plants. Black medic also has deep roots which help aerate the soil.
So before getting back to weeding, first figure out what weeds you have. References on weeds, wildflower guides, even the internet, are good places to start. Then consider what they may be telling about your soil, and if it needs improving. If these weeds can be managed to not spread, even consider leaving them, if they can help improve your soil. You then have a great educational opportunity on "weed ecology" if neighbors and visitors consider your garden "weedy"! Best of all, you'll have more time for other gardening pursuits other than weeding.