Weeds are placed in broad groups according to their life cycle; i.e. - winter or summer annual, biennial or perennial, and morphological characteristics - monocot or dicot. There are other ways that weeds can be grouped which may help turfgrass managers devise programs that will provide more effective control.

How They Carry On The Process of Photosynthesis:

All green plants carry on photosynthesis - the process whereby plants capture sunlight energy and convert it into a usable form. Photosynthesis consists of two main processes. The first is a light-dependent process whereby the light energy is converted into a biologically useful energy called ATP (adenosine triphosphate). The second, or dark (known as light independent), step is the storage of this energy into the chemical bonds of sugars and carbohydrates. Central to the light dependent reaction is the fixing of carbon dioxide (CO2) known as the Calvin, or C3 cycle. This cycle is given the C3 term because of the production of 3-carbon compounds. The cool-season grasses (Kentucky blue, perennial rye, etc.) are referred to as C3 plants.

Some plants have been found to fix carbon dioxide differently. These are mostly tropical or warm-season plants, that adds another step in the fixation process, known as the PEP (phosphoenolpyruvate) step. Instead of 3 cycles then, the carbon fixing goes through 4. Hence, these plants are know as C4 plants. Buffalograss, native to our region is an example of a C4 plant. Others would be Bermuda grass and St. Augustine grass.

Just what does this tidy, abbreviated, study mean to the turfgrass manager in controlling weeds? Simply that the C3 plants will fix carbon more efficiently at moderate or cooler temperatures than their C4 counterparts, and conversely, the C4 plants will fix carbon more efficiently at higher temperatures than their C3 counterparts. Knowing this efficiency status of these two types of plants can lead to fairly predictable outcomes in management strategies if properly implemented. For example, disturbing the soil in midsummer when the temperatures are in the upper 80's or 90's will put Kentucky bluegrass at a disadvantage against the likes of crabgrass, goosegrass, and foxtail which are all C4 plants - effectively out-fixing carbon dioxide more efficiently than the Kentucky blue at the high light and temperature season. In a nutshell, the Kentucky bluegrass is losing the battle of energy fixation in this setting. Move to the shade of a large tree, and the opposite is true. The competitive advantage shifts to the Kentucky bluegrass over that of the C4 weeds. Crabgrass and foxtails will be weaker and lower in number in shaded environments. Attempt to grow a warm-season grass in the shade and the competitive pressure from weeds will not come from C4 plants, but the C3 species.

This information should provide us with a clue as to what needs to be done in pre-emergent herbicide application. Not as much is needed in shade where the concern is controlling crabgrass, foxtail, or barnyard grass, and perhaps a heavier dose is required along curbing and driveways where the heat gives the advantage to these weeds.

Don't know your weeds that well? Not to worry. Simply observe where the undesirable critters are growing: hot, dry conditions? Most likely a C4 plant like crabgrass. Thriving in the shade? Most likely a C3 plant like chickweed.