Weed seed germination

By Barry Tickes, UA Area Agriculture Agent

Unlike weed seed, noxious crop seed has adapted naturally over many years to survive under a variety of conditions. It has commonly taken hundreds of years to develop some of the survival characteristics that we see today.

Some weed species produce more than 200,000 seeds per plant that can remain viable for 40 years or more.

There are many variables that affect the survival and germination of weed seeds and these can differ greatly from region to region. It is important to be able to predict when and how long it will take for weed species to germinate in each region.

Temperature, water, and oxygen are the main variables that affect weed seed germination in the low deserts.

Crop seed is developed to have a high germination rate all at the same time. If weed seed germinate at the same time, it could be easily controlled and eliminated.

We conducted a trial several years ago to determine when summer annual grasses emerged in the Yuma area. Grasses started to emerge in March, reached a peak in June, but continued to emerge until October.

Some of the seed may not germinate for several years. This can vary from field to field and year to year and is different for each species. Delayed and multiple germinations are typical, however, in most weed species.

Pre-emergent herbicides normally work best when applied just before the weed seed germinates. When applied too early, they may have begun to break down or been leached below the germinating weed seed. When applied too late, they may be ineffective.

Most work only on weeds as they try to develop a root system. If the roots have already begun to develop, they may be ineffective. It is often better to be a month early than a day late.

We conducted a trial at various locations in Yuma to determine how long it took for various weed seeds to germinate after exposure to moisture. Seven summer and winter annual weeds, two grasses, and five broadleaf weeds were chosen for this study.

Weed seeds were placed in tea bags and buried one-quarter inch below the soil surface and in the seed row of newly planted lettuce fields prior to the germination irrigation. Tea bags were pulled every 24 hours and evaluated for germination.

This procedure was repeated in sprinkler and furrow irrigated fields at eight locations in Roll, Bard, the Gila Valley, and the Yuma Valley. The tests began in August and were conducted each month until February.

The summer annual weeds, Purslane and barnyard grass, began to germinate within 24 hours in August but continued to germinate until December when they took 96 hours. Pigweed took 48 hours to germinate in August and continued into December when it took 96 hours.

Winter annual weeds including Canarygrass, lambsquarters, nightshade, and Shepardspurse took longer - 72 to 168 hours.

Contact Tickes: (928) 580-9902 or btickes@ag.arizona.edu.

Question of the week

Weeds science - How do you control nutsedge in Bermuda grass?

A - Cultivation

B - Topical application of citric acid

C - There is no control.

D - Ask Barry (video).