The Annual Weeds of Summer

Posted by: Alison O'Connor, Larimer County Extension

Though we're approaching the end of the gardening season (I know...I don't want to think about it either!), most of us are still battling the sea of weeds on our gardens. As I was walking Maple the other day, I took photos of a few persistent and common annual weeds that we see in our gardens, lawns and landscapes.

It's important to note these are annual weeds and germinate in spring and complete their lifecycle in one growing season (just like petunias or geraniums). Most summer annual weeds will germinate after soil temperatures warm to above 60 degrees. This year, we had a slow start to spring...and our soils stayed cool, so we didn't see weed growth as early as normal. But once temperatures heated up--BAM! Weeds. Everywhere.

There's a few approaches to weed control. One is to pull the weeds. Persistent and regular pulling will prevent the weeds from going to flower and seed (very important) and help keep the weed seed bank at bay. I'm sure most gardeners do their share of pulling weeds throughout the year. Another control option is to use an organic mulch (wood chips, grass clippings, leaves). Granted, this isn't possible in all areas of the landscape, but a thick layer of mulch in planting beds and vegetable gardens does wonders to prevent weed growth. Most weed seeds need light and exposed soil to germinate. Take away these two things and you'll have fewer weeds.

Using chemical options is also an option. For post-emergence weeds (those that have germinated), there are only a few options in our landscape areas (far more in lawns). These options include glyphosate and some organic products (like acetic acid (vingear) and oils). These tend to be non-selective products. Glyphosate is absorbed via the foliage, which does kill the root of the plant. Acetic acid and oils are burn-down products, which means that the foliage dies back. It also means that repeat applications may be necessary.

There are pre-emergence herbicides that you can use in the early spring, such as treflan (sold in Preen), isoxaben (sold in Ferti-Lome Broadleaf Weed Control with Gallery) and corn gluten meal. All  of these products can work, but may need to be reapplied. Always follow the label on the product you plan to use. And a note about corn gluten meal: While it has VERY LIMITED, SHORT-TERM weed control capabilities, it has proven, by research, to be a much better fertilizer than herbicide.

So let's look at the Weeds of Summer:

Crabgrass (Digitaria sanguinalis)

Large crabgrass

Good ol' crabgrass. Here it is in all its glory. Other plants get called "crabgrass" but this is the one and only. Crabgrass won't germinate until soil temps are above 60 degrees. In a lawn, the best defense against crabgrass is to keep the lawn healthy (well-watered and fertilized). In this situation, crabgrass found a great place to grow in rock mulch along the sidewalk. It's nice and hot and ideal for growth. Crabgrass has a very distinct seedhead, which is finger-like (hence the genus Digitaria).

Finger-like seedhead of crabgrass.

Prostrate spurge (Euphorbia maculata)

Prostrate spurge

Prostrate spurge has had a very good summer. It's everywhere. This mat-forming weed can reach impressive size by late summer if left uncontrolled. Fortunately, it's easy to pull. The one caution is that like other plants in the Euphorbia genus, the plant has a milky sap, which oozes when stems are broken. The sap can be caustic and an irritant. If you have sensitive skin, wear gloves and be sure to wash your hands following weeding.

White latex sap of prostrate spurge

Close up of prostrate spurge...you can make out the teeny-tiny flowers, which all form viable seeds!

Purslane (Portulaca oleracea)

Purslane is a pretty mild weed...it's edible, after all! And I haven't seen as much purslane this summer, but I might not be looking that hard. Purslane can be confused with spurge, but there is no milky sap...and this weed has thick, red stems. It's a succulent, so it can tolerate dry conditions with occasional moisture. I see it most often in my vegetable garden, near the drip irrigation lines.

A very mature purslane plant.

Puncturevine (Goatheads) (Tribulus terrestris)

Ugh! By far the WORST of the summer annuals. This beastly plant is just nasty and mean. If you aren't familiar with puncturevine, consider yourself lucky. The seedheads can pop bicycle tires, injure dogs' paws and wreck havoc on bare feet. This is one weed that you should 100% remove and prevent from growing. It can reach widths of feet if left to grow undisturbed...yikes!

One (yes one!) puncturevine plant.

That same plant pulled from the ground.

Puncturevine has yellow flowers and the seedheads start off green, hardening into sharp, pointed, vicious seedheads by late summer.

Yellow flowers of puncturevine.

Nasty, mean and cruel seedheads. The plant is called "goathead" because the seedhead resembles a goat's head.

And poor Maple, my faithful beagle pal and gardening buddy, was afflicted by a stray puncturevine seed on our walk. Poor thing hobbled for a few feet before sitting down, lifting her paw in the air. It's because of this that I just despise this weed. So pull it already!

Maple's paw with puncturevine seed stuck in the pad.

There are many other weeds of summer that I didn't mention, like kochia, but we can cover those another time. Happy late summer weeding, folks!