Are weeds good indicator plants? Does the presence of a particular weeds tell you something about your soil? There are many claims online that you can tell the pH of your soil by checking the weeds. Some claim that weeds indicate an imbalance in the ratio of bacteria to fungi, and other claim they can be used to detect nutrient deficiencies. Let’s separate myth from fact.
Key Takeaways
- Weeds are much more versatile than most people think and many are not always good indicator plants.
- Indicator plants can tell you about your soil, but they can difficult to read.
What is a Weed?
One of the problems with this topic is that any conclusion depends very much on the definition of a weed, and that word that is not well defined. Many consider any plant a weed when it is growing where it is not wanted. That means that just about any plant can be a weed.
Basic Plant Biology
Every plant has a preferred set of conditions where it grows best. If we limit the discussion to soil then parameters such as moisture, compaction, drainage and nutrient levels all impact how well a plant grows. Some plants are specialists which is another word for prima donnas and they have very exacting requirements. Others will grow just about anywhere.
Finding a specialist weed growing in your garden would give you some insights into the soil conditions, provided you know what they want. But are weeds prima donnas? Most of them are considered weeds because they grow in a lot of places.
Examples of Indicator Weeds
Here are some weeds that are claimed to be indicator plants.
| Name of the Weed | Indicated Condition |
|---|---|
| Field horsetail (Equisetum arvense) | poorly drained, low pH soils |
| Quackgrass (Elytrigia repens) | improper iron-manganese ratio |
| redoot pigweed (Amaranthus retroflexus) | iron-manganese ratio is out of balance. It may indicate there is too much iron or too little manganese. Also soil that is very high in potassium and manganese and low in phosphorus and calcium |
| Burdock (Artium lappa) | indicates low calcium, high potassium soils |
| Oxeye daisy (Chrysanthemum leucanthemum) | soils that are low phosphorus, high potassium and high magnesium soils |
| Legumes (clover, vetch) | low nitrogen levels |
How valid are these claims? I have battled with field horsetail in my alkaline soil (pH = 7.4) and it is really hard to get rid of. If it prefers low pH soil it should have been easy to eradicate.
If horsetail grows well at pH of 7.4, is it really an indicator plant for acidic soil?
Have you ever tried to get rid of quackgrass (quackgrass vs crab grass)? It is a tough plant that seems to grow everywhere. If it really indicated an iron-magnesium imbalance, would it not be easy to eradicate it by adding either iron or magnesium to soil? I never see that recommendation.
Legumes can fix their own nitrogen and therefore are able to grow in low nitrogen soil, but only if the right rhizobium nitrogen-fixing bacteria are also in the soil. The other fact that people forget is that legumes also grow well in high nitrogen soil in which case they don’t form nodules to fix nitrogen.
Statements such as these are also common. “Redroot pigweed can indicate too much iron or too little manganese, but it may also indicate high potassium and manganese and low phosphorous and calcium, and is often an indicator of fertile soil.” Does that really help the gardener?
Weeds That Indicate Acidic Soil
Here is a list of plants that are claimed to indicate acidic soil.
- Buttercup*
- Daisy*
- Dandelion*
- Horsetail*
- Knotweed
- Moss*
- Mullein*
- Plantain*
- Sheep Sorrel
- Stinging Nettle
My soil has a pH of 7.4 and all of the starred plants grow quite well in my garden. Have you ever heard a gardener say something like, ‘I don’t have dandelions because I have alkaline soil”? Almost everyone in North America has dandelions regardless of soil pH.
As I read through numerous government extension publications I noticed that the list of weeds that grow in acidic conditions was not consistent from site to site. The experts don’t agree on which weeds should be on the list.
The Science Supporting the Concept of Indicator Weeds
A review published in 1977 had this to say, “The information on weed indicator species is poorly documented, much of it residing only in the minds of observant farmers and gardeners.”
There are numerous studies that look at weeds and ways to control them, but I did not find any newer studies that looked at how reliable weeds are for determining soil conditions.
Limitations in What Weeds Can Tell Us
Reading the weeds and their growing conditions is much more complex than many writers make it out to be. Consider the following complexities.
Plants differ enormously in their ability to tolerate changes in soil properties. Some have a narrow tolerance and others have a wide tolerance. Many of our common weeds have a wide tolerance – that is why they are common. This tolerance is also different for each soil property. For example a weed might require a narrow range of pH, but be able to tolerate a wide range of moisture.
They are also sensitive to several environmental factors at the same time. It is therefore important to know all the factors that a particular plant is responding to. That combination of factors is not available from published lists which normally only focus on one parameter.
Perennial weeds are generally more tolerant of a condition and may be better indicators than annuals. Weed communities are better indicators than individual plants.
How well a weed grows may be more telling about soil conditions than the actual presence of the weed. A well developed dark green legume may indicate the presence of higher levels of nitrogen, even if it’s presence normally indicates a low nitrogen condition.
Do Weeds Indicate Poor Soil?
Weeds are opportunistic and grow where other things are not growing. Plants in poor soil don’t grow well and make it easy for weeds to get established because they grow better in poor soil.
However, that does not mean these same plants won’t grow in good soil. Anyone who has ever had good soil knows full well that it does not prevent weeds from growing. Weeds that are able to grow in poor soil also seem to thrive in good soil.
There are also claims that having the wrong fungal to bacteria ratio affects weed growth. Change the F:B ratio and weeds disappear.
The existence of weeds is not an indication of poor soil but certain weeds will be more prevalent in poor soil.
Indicator Plant Myths
Moss in Lawns
Moss growing in lawns is a common problem and its existence is usually attributed to acidic soil or wet, shady areas. The reality is that moss grows at various pH values and in both wet and dry conditions. There are many species of moss.
Moss is one of those plants that can grow in spots that are inhospitable to other plants, including:
- Too much shade
- Too wet
- Compacted soil (ie lack of air in the soil)
- Low fertility
Rather than being an indicator of those conditions it shows that other plants can’t tolerate the condition.
Find out more about getting moss out of your lawn.
Blue Hydrangea
A common claim is that the flower color of Hydrangea macrophylla is based on soil pH. They are blue in acidic soil and pink in alkaline soil. Although this is somewhat correct, the real cause for the color change is the level of aluminum in the soil. If you have soil that contains no aluminum, you will never have blue flowers, no matter what the pH is.
Clover in Lawns
It is believed that clover thrives in lawns because of low nitrogen levels. The reality is that clover will grow in most spots, provided it can get a foothold. Once there, it is quite aggressive and keeps expanding its territory. Clover also grows well in high nitrogen soil.
Fertilizing a lawn more produces better growing grass which can then outcompete the clover. The amount of clover will become less but not because of high nitrogen levels.
Should Gardeners Rely on Indicator Plants?
The presence of certain weed species may indicate a specific soil condition, but this is much more complex than presented in the popular press. You can’t reach any conclusion by seeing a specific weed in your garden.
Don’t rely on indicator weed lists found on the internet. In researching this post I looked at many such lists and they varied quite a bit. It is impossible to tell which ones are right and none of them provided a scientific reference.
It is easier and more reliable to use other methods for evaluating your soil condition. Here are some DIY tests you can do.
The best way to understand nutrient levels is to get a proper soil test. Home soil kits are of limited value because they are simply not accurate enough, but they are more accurate than trying to read the weeds.
Excellent! Thank you for saving me the time and energy to acquire and study perfectly useless concepts.