The Cause of Weeds in Your Lawn
Sean Breckin, AOLCP

Summer weeds are arriving on the coattails of spring weeds. Environmental conditions like drought and extreme heat make it difficult for cool-season turf grasses to compete with summer weeds. When looking at the cause of weeds in your lawn, fear not! There are underlying soil conditions to address and cultural practices to help mitigate. As you scout lawns and athletic fields, it’s important to ask: “Why is this weed here?”. When evaluating areas, different weeds tell different stories of what’s going on with the soil. Review our list of common spring and early summer weeds to understand why they’re there.

Low pH

Clover…a rapid spreader!

Clover thrives in soils with low pH, which is also an indicator of low fertility for turf grass. When identifying this plant, look for the rounded leaves that distinguish it from black medic and oxalis. This Nitrogen fixing plant does indeed provide bioavailable nitrogen to turf grass through its ability to convert atmospheric Nitrogen (N2) into Nitrates (NO3-) and Nitrites (NO2-) and Ammonium (NH4+). However, it can be problematic for athletic fields as it compromises the integrity of the playing surface. While this pollinating plant may be just fine on a homeowner lawn, it doesn’t suit athletes playing sports. This is due to inconsistent surfaces that interrupt ball rolling or cleat traction. Be sure to heavily overseed in order to keep playing surfaces consistent and safe.

High pH

Wood Sorrel (aka Oxalis…aka Shamrock)

Wood sorrel is a common perennial weed found in soils with a high pH. Keep in mind that clover, black medic, and wood sorrel can all be confused for one another and are all found in nutrient deficient soils. Therefore, it’s critical to identify differences as they indicate different soil characteristics. Wood sorrel has a heart-shaped leaf structure. If you have trouble distinguishing the trifoliate leaves, look instead at the flowering body of the plant. A bright, 5 petaled, yellow flower makes it easy to identify wood sorrel. In addition, this indicates a high likelihood of needing to add sulfur into your program – which can be confirmed with a soil test.

Nutrient Deficient Soil

Black Medic

This summer annual is a legume. Therefore, it has the ability to ‘fix’ nitrogen which is why it is capable of out competing turf in nutrient deficient soils. It has a deep central tap root that keeps it well anchored and helps it establish itself in an area of turf. Additionally, the large taproot helps it thrive in dry soils. It has a similar leaf shape to wood sorrel and clover, so be mindful of specific characteristics when scouting. Black medic flowers form yellow clusters that are replaced with black seed pods as it reaches reproductive maturity. If you’re seeing a high volume of black medic, you likely need to include more PJC ProHealthy Turf fertilizers in your program.

Moist Soil

Ground Ivy

Perhaps the most difficult weed to control in a lawn. This perennial spreads through stolons and seeds. While it can tolerate sunny areas, ground ivy does best in the shade and prefers moist disturbed sites. Mechanical removal is your best bet for decreasing the ground ivy population. In addition, be sure to overseed with an appropriate seed variety. Fine fescues and creeping fescues are going to do best competing with shady weeds.


A small summer annual that spreads rapidly through seeds that quickly germinate. While it can be seen in drier soils, carpetweed is typically an indicator of poorly drained, wet soils. Carpetweed does not compete well with established turf. On the contrary, it is an opportunist in thin turf to bare soils. Therefore, it’s important to be on top of lawn repairs to avoid any establishment of this pesky weed.

Compacted Soil


The most common summer annual comes in two forms: large (hairy) and small (smooth). Crabgrass germination is typically from midspring to midsummer. Flowering and seeding is aggressive from midsummer all the way to the first frost. Furthermore, crabgrasses can tolerate high temperatures and dry, compacted soils better than turf grass varieties. All of these factors make it extremely difficult to manage this weed in your lawn. Although, crabgrass cannot tolerate shaded sites both from trees and dense turf. So when managing crabgrass organically, be sure to: mow high, judiciously water, aerate compacted areas and overseed with a turf type tall fescue dominant blend.


A late summer annual that is often mistaken for crabgrass thrives in heavily compacted soils. Although, it germinates 4 to 6 weeks after crabgrass, which can make it easier to distinguish when doing your scouting. Goosegrass is adaptable to a wide range of site conditions and can do well in nutrient deficient soil, dry soils, landscape beds and turf stands making it difficult to eradicate. A tall turf stand is essential in managing goosegrass due to its low growth habit. Its smooth blade structure makes for a very poor playing surface on athletic fields. 

Prostrate Knotweed

Typically the first broadleaf summer annual to germinate in the spring. Prostrate knotweed is a classic indicator of compacted soils. The quick spreading plant forms dense mats that are extremely harmful to turf, as it makes it nearly impossible for turf plants to emerge. An aggressive aeration and overseed strategy is essential when dealing with prostrate knotweed. I’d recommend at least two aeration and overseeds a year—once in spring and once in late summer/fall—to re-establish a turf stand. If resources allow, and it’s an important athletic surface, I’d even add in slice seed to damage the knotweed branches and put seed directly into the soil.  


A telltale sign of compaction. Plantain has strong taproots that can ‘break’ poorly structured soils and steal calcium. Turf grass roots don’t have the moxie to break compacted soils and require assistance through aeration and overseeding and gypsum to provide bioavailable calcium and improve soil structure. It tolerates a wide range of moisture conditions making it extremely difficult to manage, especially on athletic fields where foot traffic from athletes is so high. This weed will start to show significant formation late spring and early summer.

Practical Approach

In light of the cause of weeds in your lawn, the commonality in managing weeds is growing lush turf with our practical approach and proven products. A high height of cut shades out prostrate growing weeds and limits weed seeds from getting to the soil surface. Deep watering encourages turf root growth and limits shallow, moist soils that certain weeds thrive in. Keeping pH in range through liming is essential for nutrient availability and soil structure. 

Turf species diversity is also critical to enhancing competition on athletic fields and lawns. In Spring, Rye grass is going to be most prevalent. In early Summer, tall fescues and Kentucky blue will be most competitive. Over the heat of summer, tall fescue does a great job of remaining competitive. Then in late summer and fall, Kentucky blue and rye will dominate again. Maintain high species diversity and prioritize cultural practices to compete against weeds. 

In conclusion, when identifying weeds, be sure to address the cause of weeds in your lawn. Ask yourself “What do I need to improve in my product applications and cultural practices?”. And/or, just email me directly for the recommendations you need to manage your weeds successfully AND organically. 

Good luck out there,
Sean Breckin