The evenings this week have been great. They have been cool and mostly wind-free. A great time to spray for weeds.
I'm planning on putting in lavender next spring in an area where bindweed is going to be a problem unless I can kill those perennials this fall. I'm using a combination of glyphosate and amine 2,4-D. These chemicals are systemic and will move into the root system of this and other weeds, and hopefully cause their death.
I could also use an organic-approved product such as Scythe. Scythe burns the plant tissue it comes into contact with but will not move into the roots. I don't know of any organic-approved product that has the systemic properties of 2,4-D or glyphosate.
The longer a spray remains a liquid, the more effective it is in doing its job. Sprays applied in the evening remain liquid much longer than if applied in the morning or during the day. It should never be windy when spraying as the product dries too quickly and airborne droplets will drift to other plants causing possible damage.
We all know that weeds compete with our landscape and crop plants for water, nutrients and oxygen. For that reason I want to ensure I have the majority of the weeds in this field eliminated when I plant in the spring. The new lavender plants will establish best without the competition of weeds.
In addition to bindweed, I want to eliminate the grassy weeds in this field - to me that means all grasses - and I want that grassy material, tops as well as roots, to have decomposed by spring. If I wait until spring to start to control the grasses in this area, the soil will be contaminated with allelopathic compounds, biochemicals produced by grasses and some other plants to inhibit the growth of neighboring plants.
I'm sure most of you have heard about the difficulty of growing plants under the canopy of a black walnut tree. Allelopathic compounds produced by this tree prevent other plants from growing in its vicinity. Sagebrush, sycamore trees, and many other plants produce these same compounds. I want the plant tissue responsible for releasing these compounds completely broken down by spring.
If I was using glyphosate without 2,4-D, I would be adding ammonium sulfate at the rate of 1 percent by weight to increase the uptake of glyphosate. Glyphosate is an effective grass killer and since there are perennial grasses in the field along with the broadleaf bindweed, I'm mixing glyphosate with 2,4-D. 2,4-D kills broadleaf plants. I'm using the amine form of 2,4-D as I don't want any vapors damaging the neighboring planting of lavender or other plants at this tree farm. The ester and low-volatile ester formulations of 2,4-D can vaporize and drift off site to damage other plants. If I used amine 2,4-D on the bindweed without glyphosate it would have little effect but when amine 2,4-D and glyphosate are combined, the mix has a synergistic effect greatly increasing its effectiveness.
I could use a three-way weed killer containing dicamba but I don't want any chemical remaining when I plant in the spring. Dicamba tends to hang around in the soil while the amine formulation of 2,4-D and glyphosate don't. Dicamba injury of trees and shrubs is quite common even a year after the chemical is applied. People like to eliminate the weeds in their lawns and shrub beds as quickly as possible and often use a product containing dicamba to accomplish this. Sometimes folks in the garden centers even recommend such products. I never recommend products containing dicamba due to the possible damage that can occur to non-target plants.
The 2,4-D amine formulation is much safer. It is also less effective as a weed killer than the vaporizing ester 2,4-D formulations or dicamba and therefore may need to be applied several times. Weeds that attempt to recover are sprayed again as soon as I see new growth. Obviously, I would not use the combination of glyphosate and 2,4-D on the lawn unless I was trying to kill the grass.
I could cultivate with a hoe, shovel, or other implement to control these weeds. If you are going to do this you need to keep the 4-inch rule in mind. When a perennial weed is decapitated, the plant attempts to recover. Buds on the roots start to grow and this new growth depletes the roots of some or all of its stored energy. If we allow the plant to put on four or more inches of new growth before we hoe it again, the energy taken from the roots to grow the new tissue has already been replaced by the photosynthetic activity of that 4 inches of green tissue.
So the key to weed control by cultivation is attacking the weed again before it has grown 4 inches. Some weeds have very extensive and deep root systems and it takes many cultivations (or sprays) to kill these weeds. The sooner you spray or cut the weed off again, the sooner you are going to get control of the problem. I'll still have weeds come up in this new planting next year but treatments this fall will significantly reduce weed pressure next year.
Dr. Curtis E. Swift is a retired horticulture agent with the CSU Extension. Reach him at Curtis.Swift@alumni.colostate.edu or check out his blog at http://SwiftsGardeningBlog.blogspot.com. He owns Swift Horticultural Consulting and High Altitude Lavender.