What is in your toolbox for Weeds?

By Susan Carter, Horticulture and Natural Resources, CSU Extension, Tri River Area

Spring is here especially in the valleys.  I manage 4.5 acres at our Mesa County Extension office and the weeds are off to the races for growing.  So how do we manage weeds?  First and most important is to identify the weeds or at least figure out what growth type they have such as annual, biennial, winter annual or perennial.  This will help you to come up with the best plan of attack or tool to use.

Dandelion-A perennial Photo By Susan Carter

So let’s start with the annuals.  They are plants that come up from seed from the previous year and complete their life cycle in one year or less.  Depending on the species, they may come up in early spring or they may be warmer season and come up later.  For example, there are many mustard plants that come up very early such as flixweed and blue mustard.  Mustards or plants in the Brassicaceae Family (Cruciferous was the old family name) have four petals arranged in a cross or crucifer shape.  Their seed pod is also very unique and is either a silique or silicle.  The silique faces upwards or downwards and is a long and thin.  The silicle can be heart shaped and is wider than long.  Some mustard plants are edible and others are the opposite so identification and knowing what parts to eat is very important.  These mustards set seed almost immediately.  Since they are annuals they are easy to pull, hoe or till.  But if you do not catch them right away or if you have too large an area, you could use a pre-emergent herbicide.  Pre-emergent target seeds when they are germinating.  They need to be applied prior to any germination.  So I missed the boat for the flixweed and blue mustard for pre-emergent this year so we will be pulling or hoeing or using a post emergent herbicide in large areas. 

Flixweed, an annual in the Mustard Family- Photo by Susan Carter

I should have applied the chemical in early March or even late February since winter was so warm and plants are growing in the Grand Valley.  Make sure to always read the label and follow the instructions, it is the law.   The instructions are written to have the most success so if it says apply a certain amount or to water in, follow the instructions.  Also make sure the weed you are trying to control is listed on the label as being controlled by that specific chemical.  Or if allowable in your area, you could flame the weeds.

Biennials are plants that come up from seed the first year and form a rosette or basal foliage (low to the ground).  The second year they sprout upward, flowering and going to seed.  Then the whole process starts again.  Many people don’t notice some of this type of plants till their second year.  An example of this would be prickly lettuce as in my picture below.  Ideally if you can remove or kill biennials the first year, then you are preventing more seeds from entering your soil and build that seed bank.  Certainly using a hand tool to remove the first year would be good.  Pre-emergent chemicals will also work to prevent seed germination.  And post emergent non-selective or broad leaf killers can be used.  Again, match the chemical to the specific plant being controlled.  And I must admit I hate when people spray really tall weeds.  Then you just have tall big brown dead weeds.  Mowing may slow flowering but many plants will just try to flower again by pushing a new flower head or may flower very low close to the ground.

Winter annuals like Cheatgrass (Downey Broome), Storksbill (Redstem Filaree), prickly lettuce all germinate late summer to early fall, sit there over winter, then take advantage of winter moisture and early warm temperatures and off to an early start they go.  They are among the first to go to seed.  And some like Cheatgrass become fire hazards because they dry out totally by mid-summer.  Cheatgrass is a noxious weed in all 50 states and alone has changed the fire cycle in the West.  Typical sagebrush land prior to cheatgrass would burn about every 75 years.  This interval has been shortened to as little as 5-7 years.  Much work is being done to find a control of this noxious weed.  I have had great success applying a pre-emergent in early August prior to germination, pulling when young in flower beds and using my D hoe to disturb in small patches.  Here is a factsheet on cheatgrass: http://extension.colostate.edu/topic-areas/natural-resources/cheatgrass-and-wildfire-6-310/

Cheatgrass aka Downy Broome - Photo by Susan Carter

Perennial weeds come back from the roots.  They may keep foliage above ground over winter or may die back to the ground and sprout up again from the root.  There are some beneficial insects that slow these plants down.  Bindweed mites can actually get rid of the bindweed if in unirrigated areas.  If watered, the mites cannot keep up with the growth of the plants.  Visit the Palisade Insectary website for more beneficial insects for specific weeds: https://www.colorado.gov/pacific/agconservation/biocontrol

Bindweed Photo by Susan Carter

Other tools that can help include flaming if allowed in your area.  Best not to do in very dry areas.  I believe whenever possible good old fashion labor is a great way to remove weeds.  Digging can work on some species.  But there are some weeds that have roots to China and pulling or digging can make the situation worse.  There are organic labelled herbicides like 20% vinegar, but though organic, it is still an acid and is not cheap.  It works by burning off the tops of the plant so a good control for annuals that do not return from their roots.  Another tool is to out compete the weeds which work well in lawns.  See our lawn care sheet. http://extension.colostate.edu/topic-areas/yard-garden/lawn-care-7-202/

There are also some new zappers that use LED lights on the market.  Tony, our turf specialist, said he was going to try it.  So Tony, we will wait for your review.

So in summary, get to know the bad guys in your garden and make informed decisions based on the type of weed, the amount of weed, the timing of the year and what you personally can handle or afford.  We have many more ways of tackling the weeds than mom and dad did.  And get those weeds when they are small and manageable.

Blue Mustard, an early spring Annual- Photo by Susan Carter