Puncturevine, Tribulus terrestris, a List C Noxious Weed in Colorado. (Photo: CSU Extension).

With the recent moisture and the heat of summer, summer annual weeds have sprouted and are growing. Winter annuals have produced seed and are at the end of their life cycle. Winter annuals with seeds should be pulled and disposed of in the trash. Weeds with seeds should not be composted in a home compost system as the seeds will likely not be killed in the home composting process.

Common summer annual weeds include redroot pigweed, lambsquarter, spotted spurge, prostrate purslane, Russian thistle, kochia, knotweed, puncturevine and others. Integrated weed management techniques that work well on summer annual weeds are prevention, cultural, mechanical and chemical. The only biological control for summer annual weeds is two weevils to help manage puncturevine, Microlarinus lareynii and Microlarinus lypriformis. The weevils are available through the Colorado Department of Agriculture’s Insectary in Palisade.

If you don’t know what weeds you have it is best to have them identified before you choose your management methods. Here are some basic features that you can use to identify some of the summer annual weeds. For additional plant identification assistance, contact the CSU Extension Office.

Redroot pigweed – easily identified due to the reddish/pink color of the tap root, inconspicuous flowers

Lambsquarter – a medium height weed with somewhat triangular/arrow shaped leaves that may appear mealy, inconspicuous flowers

Spotted spurge – a low growing weed with reddish stems and spots in the middle of the leaves, oozes a milky sap when branches are broken

Prostrate purslane – a low growing weed with very moist, succulent foliage, reddish green in color

Knotweed – tends to grow in compacted areas and will come up through cracks in sidewalks and asphalt, trailing branches with swollen nodes

Puncturevine – well known for puncturing bicycle tires, spreading branches with fine leaves and small yellow flowers that produce a bur with spines, a List C noxious weed

Russian thistle – becomes one of our tumbleweeds, a very prickly tumbleweed, initially it can be mistaken for grass

Kochia – a very hairy weed that becomes a fuzzy tumbleweed, it is related to Russian thistle

Cultural and prevention techniques work for any weed species by keeping the weeds from getting established initially. Annuals don’t have enough energy to compete with healthy grasses and forbs. Preventing or reducing the number of seeds that these annuals produce is the key to reducing numbers of these weeds in the future. Mechanical methods have mixed success with summer annuals. Mowing reduces the number of seeds produced by red root pigweed, lambsquarter, kochia and Russian thistle by reducing the plant height and number of branches. However, the weeds will likely produce other branches lower than you can mow. Weeds such as purslane, spurge and puncturevine have a low growth habit that makes mowing useless. However, hoeing or hand pulling works well on these weeds. You need to be careful when hoeing or hand pulling these weeds, or they can re-root. Removing them is the best method to keep them from re-rooting. Chemical methods work well on annuals. You can use an organic herbicide or a synthetic herbicide. Organic herbicides are ones that contain acetic or capric acid (i.e. Burnout, 30% vinegar, vinegar you use for cooking is 3 – 5% acetic acid), citrus, clove or cinnamon oil or ammonium nonanoate (Biosafe product). Any organic herbicide is best used on small plants without established root systems. Larger plants take multiple applications to kill the weeds as the organic herbicides only burn and kill the foliage and do not kill the root system. Synthetic herbicides work well on summer annual weeds and do kill the root system. As always read and follow the herbicide label.

By Sharon Bokan, Colorado State University Extension, Boulder County. Sharon is the Small Acreage Coordinator at Colorado State University Extension Boulder County. For more information, call 303.678.6176, e-mail sbokan@bouldercounty.org or visit ext.colostate.edu/boulder.