The story on annual weeds in the landscape is the story of the weather. Every vacant lot shows the nicer than average weather last spring. Remembering back to the spring and early summer wet weather explains the size and density of weeds now.

Recent weather helps explain why so few new weeds are seen now.

Annual weeds in the desert come in two distinct groupings. Some sprout in the spring, grow in the summer and fall and seed then die in early winter. These are called summer annual weeds.

A mirror image happens for the other group. These weeds begin to sprout in the fall and grow through the winter. In late spring they flower, seed and then die in early summer. These seeds lay in the soil now waiting for good conditions to get started. This cylce depicts the winter annual weeds.

Recently the weather has been a Camber of Commerce advertising: Warm sunny days and clear cool nights. From the perspective of a seed deposited last summer waiting to sprout in the fall its been a longer than normal wait.

The primary conditions to trigger sprouting include soil temperature, day length and moisture. The soil temperatures are correct. Since the vernal equinox the day lengths shorten to the liking of the seeds. What limits the onset of seed sprouting this fall is the lack of rain to wet the soil and activate germination.

Until rains come most of the weed seeds will delay their sprouting. Oddly, even in complete drought years a few seeds always sprout attempting to carry on the species. Once the winter rains arrive then will the ground will become spootted with newly emerged seedlings.

Years with extra rain the plant densities increase greatly to the point where most hills take on the appearance of a Chia Pets.

The weather this fall delaying the weed pressures should be viewed as a double edged sword. For weed control the fewer the weeds the easier the control. It also means there is still time to apply a pre-emergent weed control product and get great results.

The other edge of the sword means the delay or reduction in numbers of the annual desert wildflowers. Warm dry falls and winters are nice when thinking of weeds but may lead to a dissapointing spring wildflower show.

As a note, the weeds seen now are old and going to seed. This completes their cycle that began when the seeds sprouted in spring. As with all annuals the "swan's song" of death comes with seed production. Seeds to lay tucked in the soil waiting for the right conditions to sprout next year. Removing any plants with their seed heads will reduce the number of seeds for next year.

I need to make a correction from last week's column. The South Mountain Farmers' Market at 6414 S. Southern at Waldron Farms is open on Saturdays not Sundays from 9 am to 1 pm. Please note that and I hope it did not cause too much frustration.

Written by Terry Mikel, College of Agriculture and Life Sciences, the University of Arizona, 602-470-8086.
Material originally appeared in Arizona Republic
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