What's bugging you? July 6, 2016

By Jim Leser


Weeds seem to be my bane this year. Not only have I failed to get on top of my weeding chores but recent rains will surely add more to my misery. Maybe you find yourself in the same boat.

Controlling weeds in our landscapes follows three important approaches. These are prevention, cultural control and the use of post-emergent herbicides.

Prevention could actually include both chemical and cultural methods. We can keep weed problems minimized through the use of weed barriers and mulches. Keep in mind that many plants have seeds that prefer to germinate in gravelly areas rather than in bark mulches. Also, over time weed barrier fabric will collect soil on top, providing a medium for weed seeds to germinate and grow.

Pre-emergent herbicides are another tool for weed prevention. Weed seed can blow in from nearby properties, can be carried by surface water during heavy rains, or can be introduced by applications of contaminated organic matter like compost or manure. But oftentimes we provide our own weed seed problems by not controlling weeds before they produce seed.

There are both summer annuals whose seed germinates in the spring and summer and winter annuals whose seed germinates in late summer and fall. Obviously your pre-emergent herbicide application should be timed to stop these weeds from emerging, either a fall application for weeds such as cheat grass, or a spring application for weeds such as mustards and dandelions.

Contrary to popular belief, pre-emergent herbicides do not stop seed germination but rather kill the seedling as it penetrates the chemical barrier when it attempts to emerge from the soil. These chemicals will need to be watered in to be activated to form this barrier.

I guess cultural control of weeds would include hoeing out your weeds before they flower and produce seed. But what I am really thinking of is by maintaining a healthy garden or lawn, weeds have more difficulty becoming established. Bare soil is to be avoided. Also, watering deeply but less frequently will hurt shallow-rooted weeds. And as mentioned earlier, mulches are your friends when it comes to weed prevention. Lastly, mowing and catching the seed loaded weeds will help as a last resort.

Once weeds have emerged above ground they must be handled in other ways. This applies to annuals, perennials and biennials. And perennials cannot be controlled through the use of pre-emergent herbicides. Those with extensive root systems like bindweed and whitetop (hoary cress) cannot be controlled through hand-weeding either.

Post-emergent herbicides containing 2,4-D offer broadleaf control without affecting grasses. Glysophate containing herbicides are non-selective, killing all plants on contact. Make sure you read the label before using these herbicides. Off-target damage to desirable plants can occur if you don't follow the label.

Some weed species produce thousands to hundreds of thousands of seed. And this seed can stay viable in the soil often for decades. My advice, prevention is the most effective route to a sane weed management strategy. If only I had heeded my own words!

Jim Leser retired to Cedaredge in 2007 after a career with Texas A&M University Extension in entomology. He is a member of the Cedaredge Tree Board and a Colorado Master Gardener.