Weeds are the backbreaking drudgery of the summer gardener’s existence. They seem to burst from the soil the second we turn our back on the garden. While we wage war to prevent them from overtaking the tomato patch, it’s easy to think of weeds as the enemy. But if you learn to listen, you’ll realize that all those weeds are whispering. The type and prevalence of certain weeds in our gardens can tell us things about our soil composition, fertility, and pH.

When we learn to listen to weeds, we can work to improve our soil.

Compacted Soil

Compacted soil occurs when soil particles are pressed together, allowing few air spaces. This type of soil can be difficult for many plants to get the air, water, nutrients, and space they need for their root systems. Heavy clay soils are prone to compaction, but foot traffic, garden equipment, and even heavy rainfall can all contribute to compaction.

Several weed species are adapted to take advantage of compacted soils, allowing them to grow in an opening where many other plants can’t thrive. Some of these weeds, like burdock, feature large, tough taproots that can grow through hard soil. Others, like chickweed, have shallow root systems that easily penetrate just the surface of the soil.

Here are some weeds that commonly grow in compacted soil:

• Dandelion (Taraxacum spp.)
• Dock (Rumex spp.)
• Chicory (Chicorium sp.)
• Chickweed (Stellaria media)
• Plantain (Plantago spp.)
• Pineappleweed (Matricaria discoidea)
• Knotweed (Polygonum spp.)

What can you do about compacted soil?

Adding organic matter is a great way to improve compacted soil. Try adding a few inches of finished compost to each bed. Cover crops are another great idea. They add organic matter and some, like daikon radishes, also help break up hard pans. You can also use a garden fork or broad fork to help lift and loosen soil by hand.

To prevent further compaction, avoid walking on garden beds and opt for no-till methods when possible.

High Fertility

Some weeds are more prevalent when certain nutrients like phosphorus, magnesium, or nitrogen are available in high quantities. These nutrient imbalances can occur naturally or as a result of over fertilization.

Here are some weeds that can indicate a high nutrient level:

• Lambsquarters – High Nitrogen
• Pigweed (Amaranthus spp.) – High Nitrogen
• Purslane (Portulaca oleracea) – High Phosphorus
• Ostrich fern (Matteuccia struthiopteris) – Rich Soil
• Wood sorrel (Oxalis spp.) – High Magnesium
• Mustard (Brassica spp.) – High Phosphorus
• Knapweed (Centaurea spp.) – High Potassium
• Henbit (Lamium amplexicaule) – High Nitrogen

What should you do about high fertility?

You’re probably thinking something along the lines of, “wait, don’t I want my soil to be fertile?” Yes, to a certain extent, but we also want our soils to be balanced. An excessive amount of any nutrient can cause more issues than it solves. For example, excessive nitrogen can prevent the uptake of essential micronutrients like iron, manganese, and zinc. It can also encourage plants to put on tons of foliar growth at the expense of fruit growth. You could end up with very lush looking plants that don’t produce well.

Excess nutrients can also end up in local watersheds with devastating results. Excess phosphorus, for example, is often washed away with rainwater where it ends up in ditches, streams, and storm drains. It eventually finds its way to rivers, lakes, and oceans where it causes toxic algal blooms make the water unsafe for humans and wildlife.

To prevent excess nutrients, have your soil tested before fertilizing or adding amendments. When possible, use well-balanced natural alternatives like compost instead of chemical fertilizers.

Put fertility to good use by planting beds with heavy feeding crops like corn, broccoli, and tomatoes.

Low Fertility

There are also many weeds that take advantage of low fertility. These thrifty weeds will flourish in nutrient-poor soils where even our toughest vegetables will struggle.

Here are some weeds that can indicate low fertility:

• Ragweed (Ambrosia spp.)
• Mullein (Verbascum thapsus)
• Crabgrass (Digitaria spp.)
• White clover (Trifolium repens)
• Plantain (Plantago spp.)

What should you do about low fertility?

One immediate solution is to top dress your garden in a couple inches of finished compost. Another great way to sustainably add more fertility to your soil over time is through nitrogen-fixing cover crops and green manures.

We also recommend getting a soil test. A good test will help you identify specific nutrients that your soil is lacking and allow you to amend your soil without wasting money and resources.

If you’re working on a pollinator or native plant garden, you can also select plants that are well adapted to low fertility. These include, coreopsis (Coreopsis spp.), goldenrod (Solidago spp.), yarrow (Achillea spp.), and Ninebark (Physocarpus opulifolius).  

Acidic Soil

Acidic soil naturally occurs in wooded areas and is common throughout the eastern United States. While many farms on the east coast have naturally acidic soils, most vegetable crops grow well in neutral to slightly acidic soil.

Most food crops do best when the soil pH is between 6.0 to 7.0. While some will tolerate soil that leans more acidic, others like beets will offer a very poor yield. Soil that’s too acidic can prevent plants from absorbing key nutrients like calcium, magnesium, and potassium.

Here are some weeds that may indicate acidic soil:

• Mullein (Verbascum thapsus)
• Ox-eye daisies (Leucanthemum vulgare)
• Sheep sorrel (Rumex acetosella)
• Moss
• Horsetail (Equisetum spp.)
• Knotweed (Polygonum spp.)
• Hawkweed (Hieracium spp.)

What should you do about acidic soil?

A soil test is always a good idea to confirm your suspicions. Thankfully, acidic soil is relatively cheap and easy to amend. To neutralize acidity in your soil, spread agricultural lime (finely ground limestone) over your beds. If you have a soil test, look at the lime application rates to based on your soil pH to find the appropriate amount for your garden.

If you have a particularly acidic area, you can also take advantage of it and plant blueberries or ornamentals like azaleas, both of which thrive in acidic soil.

Alkaline Soil

Alkaline soil isn’t naturally common in the eastern United States. It’s common throughout much of the Midwest and western half of the country (excluding the Pacific Northwest). However, the excessive use of lime without a soil test and proper application rates can lead to soil that’s too alkaline. Alkaline soil can prevent your crops from taking up important nutrients like iron, manganese, and zinc.

Here are a few of the plants that may indicate alkaline soil:

• Chicory (Chicorium sp.)
• Chickweed (Stellaria media)
• Queen Anne’s Lace (Daucus carota)

What should you do about alkaline soil?

If a soil test confirms that you have alkaline soils, you can adjust the pH level by amending the soil with sulfur. Like lime, it’s a relatively affordable and safe garden amendment. You can also use acidic amendments like peat and pine needles to help maintain and encourage a more acidic pH over time for acidity loving crops like blueberries.