Weeds as Environmental Indicators

Weeds can help us learn to read our country. Why are the ‘weeds’ there? Notice which ones are abundant. Are they robust? What are they telling us about our soil? Nutrient status, moisture, soil type? While many weeds may be used as indicators, some, like Shepherd’s Purse (Capsella bursapastoris) will grow on most soil types and so are not reliable indicators. 

What kind of disturbance has occurred? Is it ongoing? Episodic? Sometimes soil conditions have changed dramatically because of the disturbance. Clearing of upper forested slope will change the downslope hydrology, sometimes leading to a waterlogged condition downslope. Sometimes the soil will become dry and lacking in humus because of exposure. Erosion can remove the top soil layers leaving a clay base. Chemical and microbiological conditions may change. In most cases, the weeds are purely indicators and, as early stages of succession, will help to identify the ways in which soil biology and chemistry is changing over time. There may be a different suite of weeds in a wet year to those in a dry year.

Identifying soil conditions and seasonal changes will be particularly helpful if planting is being undertaken.

Most information readily available is for North American species. Keep an eye out for weed distribution on your land and for changes in time and by season.


Weed characteristics which make them useful indicators

A useful article is Weeds as Indicators of Soil Conditions by Stuart B. Hill and Jennifer Ramsay at http://eap.mcgill.ca/publications/EAP67.htm

The characteristics they list include:

  1. Deep penetration by their roots often enables weeds to accumulate various elements from the subsoil, particularly trace elements, and transport them to the soil surface Different “accumulator” plants concentrate different elements
  2. Weeds have also been used as indicators of the presence and quality of ground water
  3. The primary value of weeds is their ability to reveal information about the properties of our soils, particularly their nutritional status, pH, and presence of a hardpan.
  4. Limits of tolerance to environmental factors vary. Some weeds may have a narrow tolerance for one variable but a wide tolerance for others. The best indicators are those with narrow tolerances because they would only be found associated with specific conditions.
  5. Plants may be sensitive to several environmental factors
  6. Perennial weeds often make better indicators than annuals
  7. Weed communities are better indicators than single species
  8. Growth characteristics of a weed may be as revealing as its presence

What weeds are associated with what types of soils?

An acidic soil is a soil with a pH below 7.0.  Look for these weeds as an indicator of an acidic soil: Bentgrasses (Agrostis species), Bracken (Pteridium esculentum), Corn Spurry (Spergula arvensis), Curly Dock (Rumex crispus),  Dandelion (Taraxacum officinale), Knapweeds  (Centaurea species), Mayweed (Anthemis cotula), Mosses (Musci class), Common Mullein (Verbascum thapsis),  Nettles (Urtica dioica), Orange Hawkweed (Hieracium aurantiacum), Ox-Eye Daisy (Leucanthemum vulgare), Plantain (Plantago major),  Prostrate Knotweed  (Polygonum aviculare), Sheep Sorrel (Rumex acetosella), Sow Thistle (Sonchus species), Wild Radish (Raphanus raphanistrum), and Wild Strawberries (Duchesnea indica).

Alkaline soil has a pH higher than 7.0.  Weeds that indicate an alkaline soil are: Bladder Campion (Silene species), Field Peppergrass (Lepidium virginicum), Goosefoot (Chenopodium species), Nodding Thistle (Carduus nutans), Pennycress (Thlaspi arvense), Scarlet Pimpernel (Anagallis arvensis).

A healthy, fertile soil will have a pH of 6.2 to 7.0.  Weeds indicating a fertile soil are: Bentgrasses (Agrostis species), Chickweed (Stellaria media), Chicory (Cichorium intybus), Dandelion (Taraxacum officinale), Groundsel (Senecio vulgaris), Henbit (Lamium amplexicaule), Lamb’s-Quarters/Fat Hen (Chenopodium album), Pigweeds (family Amaranthaceae), Purslane (Portulaca oleracea), Lantana (by growth form; Lantana camara).

Low fertility, depleted soil: Bladey Grass (Imperata cylindrica), Bracken (Pteridium esculentum), Plantains (Plantago species), Sheep Sorrel (Rumex acetosella), White Clover (Trifolium repens), Whisky Grass (Andropogon virginicus), Wild Radish (Raphanus raphanistrum).

A dry and exposed soil encourages annuals. Fireweed (Senecio madagascarensis), St John’s Wort (Hypericum perforatum), Virginia Pepperweed (Lepidium virginicum), Yarrow (Achillea millefolium).

A heavy or clay soil may have Broadleaf Dock (Rumex obtusifolius), Canada Thistle (Cirsium arvense), Chicory (Cichorium intybus), Common Chickweed (Stellaria media), Creeping Buttercup (Ranunculus repens), Crowsfoot Grass (Elusine indica), Dandelion (Taraxacum officinale), Cat’s Ear (Hypochaeris radicata), Knotweed (Polygonum aviculare), Milkweed (Asclepius species), Mouse-ear Chickweed (Cerastium glomeratum), Plantain (Plantago major), and Wild Garlic (Allium vineale). 

Weeds that indicate a wet, poorly draining soil are:  Mosses (all species), Buttercups (Ranunculus species), sedges (Carex spp), Rushes (Juncus spp.), Smartweeds (Persicaria spp.), Canada Goldenrod (Solidago canadensis), Common chickweed (Stellaria media), Crowsfoot Grass (Eleusine species), Docks (Rumex spp.), Mouse-ear Chickweed (Cerastium glomeratum), Ox-Eye Daisy (Leucanthemum vulgare), Poison Hemlock (Conium maculatum), Sheep Sorrel (Rumex acetosella), Stinging Nettles (Urtica urens), Yellow Nutsedge (Cyperus esculentus). These plants indicate that at least for some part of the year, the area will be swampy though it may dry out.

Weeds that grow in sandy soils are: Coral Berry (Ardisia crenata), Goldenrods (Solidago spp.), Sandburrs (Cenchrus spp.), Small Nettle (Urtica urens), Whiskey Grass (Adropogon virginicus).

Weeds that indicate a hardpan soil include those with deep tap roots (Dock, Dandelion, Cat’s Ear, Thistle), Field Mustard (Brassica nigra), Morning Glory (Ipomoea purpurea), Pennycress (Thlaspi arvense), and Quack Grass (Agropyron repens). 

Previously cultivated soil: Lamb’s-quarters (Chenopodium album), Plantain (several species), Ragweed (Ambrosia artemisiifolia), Purslane (Portulaca oleracea), Dandelion (Taraxacum officinale), Chickweed (Stellaria media), Pigweeds (family Amaranthaceae).

Shaded soil: Common chickweed (Stellaria media), Mouse-ear Chickweed (Cerastium glomeratum), Wandering Jew (Tradescantia fluminensis, T.zebrina).

Indications of nutrients, metals, etc. that some weeds show

Individual weeds that indicate a soil’s nutrient values are useful in determining conditions.

  • Annual Bluegrass (Poa annua) indicates very low calcium, low humus, low bacterial count, and high magnesium levels. 
  • Common Chickweed and Mouse Ear Chickweed indicate very low calcium and phosphorus levels, and very high potassium and sodium levels. 
  • Crabgrass indicates very low levels of calcium and phosphorus, low pH, low humus, very high chlorine levels, and high levels of magnesium and potassium.
  • Dandelions indicate very low levels of calcium, and very high levels of chlorine and potassium.
  • Oxalis indicate very low levels of calcium and high levels of magnesium.
  • Purslane and Mustard indicate an abundance of phosphorus. 
  • Red Clover indicates an excess of potassium. 
  • White Clover indicates very high levels in chlorine, magnesium, and sodium. 
  • Wild Garlic indicates very low calcium and bacterial count, and very high levels of chlorine, magnesium, potassium, and sodium. 
  • Thistles – high nitrogen.
  • Milk Thistle – potassium.
  • Nitrogen fixing plants like Clover and other peas, Bracken – low nitrogen.

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