With the warmer than normal winter, many plants are emerging early, especially weeds. What is a weed? Is it a plant merely growing where unwanted, or is it an undesirable foe to be removed? If an uninvited guest weed pops up in our turf or gardens, most of us are conditioned to assume that it is something to be removed. Weeds actually are trying to tell us something about the condition of our soil, acting as soil indicators. A population of weeds growing in an area can indicate whether the soil is acidic or alkaline, whether that soil is a healthy, balanced soil, or if it is nutrient deficient. Weeds can indicate a poorly draining soil, or a soil that is unable to retain moisture. So in reality, weeds have a lot to tell us if we know "how to listen" by knowing how to interpret their presence.
Some of the most common and widespread weeds are indicators of soil pH. An acidic soil has a pH below 7. The presence of both dandelion and common mullein together can indicate an acidic soil and common mullein can also mean low fertility soil. Other common weeds that are indicators of an acidic soil are curly dock, knapweed, mosses, nettles, wild viola, plantain, and wild strawberries. More desirable plants that grow well in acidic soil are azaleas, blueberries, endive, hydrangeas, rhododendrons, rhubarb, potatoes, shallots, sweet potatoes, and watermelons. Alkaline soil has a pH higher than 7. Some weeds that can indicate an alkaline soil are Roman (true) chamomile, black henbane, stink weed, nodding thistle, and wild carrot. Asparagus, broccoli, beets, lettuce, muskmelons, onions, and spinach are plants that do well in alkaline soil.
Weeds that indicate a hardpan soil are field mustard, horse nettle, morning glory, pennycress, and quack grass. Broccoli, cabbage, cauliflower, and mustards grow well in this type of soil. Previously farmed crop soil can have carpetweed, chickweed, dandelion, lamb’s quarter, plantain, purslane, ragweed, and pigweed.
Wet soil areas, while obvious during a rainy season, could appear fairly dry at other times. Weeds that indicate a wet and poorly draining soil are bindweed, bull sedge, goldenrod, cattail, coltsfoot, foxtail, horsetail, jewelweed, May apple, mosses, and stinging nettles. These weeds are excellent indicators that the area will be soggy at some time during the year.
Individual weeds that indicate a soil’s nutrient values are useful in determining if the soil is nutritionally unbalanced. Annual Bluegrass indicates very low calcium, low humus, low bacterial count, and high magnesium levels. Plantains indicate very low levels of calcium, low humus levels, and very high chloride, magnesium, potassium, and sodium. Common Chickweed indicates 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 chloride levels, and high levels of magnesium and potassium.
Dallisgrass indicates low calcium, very high magnesium, and high potassium levels. Dandelions indicate very low levels of calcium, and very high levels of chloride and potassium. Hop Clover and Oxalis indicate very low levels of calcium and high levels of magnesium. Prostrate Spurge indicates low calcium levels and very high levels of chlorine, magnesium, potassium, and sodium. Purslane and Mustard indicate an abundance of phosphorus. Red Clover indicates an excess of potassium. Wild Garlic indicates very low calcium and very high levels of chloride, magnesium, potassium, and sodium. Yarrow indicates low potassium.
While certain weeds are indeed fairly reliable indicators of growing conditions, a soil test is the surest way to confirm a soil’s deficiencies. Test results will indicate what amendments are needed for specific plants and crops. Soil test kits are inexpensive and available at the Penn State Cooperative Extension office at 670 Old Harrisburg Road. There will always be a battle between gardeners and weeds. However, weeds can have value as soil indicators, so learn what their presence may be telling you about your soil.
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