Weed species also differ in the seasonal timing of their germination and emergence. Germination of many species is governed by growing degree–days (GDD)—the summation of the number of degrees that each day’s average temperature exceeds a base temperature. This concept is founded on the assumption that, below the base temperature, the organisms (in this case seeds) are quiescent, and that as “thermal time” accumulates above this temperature, their development proceeds. In addition, some newly shed weed seeds must first undergo a period of unfavorably cold or hot conditions before they can germinate in response to favorable temperatures. This initial, or primary, dormancy delays emergence until near the beginning of the next growing season—late spring for warm-season weeds (dormancy broken by cold period over winter), and fall for winter annual weeds (dormancy broken by hot period in summer)—when emerging weeds have the greatest likelihood of completing their life cycles and setting the next generation of seed.
Remember that none of these strategies can be expected to eliminate the weed seed bank, and also that you may need to change seed bank management strategy as the seed bank itself changes. The reason the weed seed bank is so difficult to manage is because it contains not only many seeds, but many different kinds of seeds, with typically 20 to 50 different weed species in a single field. In other words, the grower may have to deal with 20 to 50 different plant survival strategies! Thus, there will almost always be some weeds that tolerate, or even thrive on, whatever combination of seed bank management strategies the farmer adopts.
Weed Seed Bank Dynamics
Organic growers aim to manage their weed seed banks in the opposite fashion from a long term savings account: minimize “deposits,” and maximize “withdrawals” (Forcella, 2003). Weed seed bank deposits include:
A little effort in understanding your weed seedbank [sic] can give you valuable information about what weeds to expect in a given growing season, weed density, and when most weed germination will take place. To get a weed preview, you can germinate weeds indoors as you’re waiting to plant. For summer annual weeds, such as velvetleaf, foxtail, lambsquarters, and pigweed, March–April is a good time to sample weed seedbanks [sic] in the North Central region. Using a soil probe or a garden trowel, take 20 samples to a 2” depth in a ‘W’ pattern from the field you’re interested in. Place the soil in a pie dish, put in a warm place (> 65 º F) and keep moist. Within one to two weeks, you should have an idea of what weeds will be emerging in your field as the soil warms.
Understanding the impact of management practices on the vertical distribution of seeds is important because it can help us predict weed emergence patterns. For example, in most soils small-seeded weeds such as kochia, Canada thistle, and common lambsquarters germinate at very shallow depths (less than ½ inch). Large seeded weeds such as common sunflower have more seed reserves and may germinate from greater depths.
The weeds present four weeks after crop sowing usually represent the most important proportion of the total weed population, at least from the standpoint of in-crop weed control. The density represented by this proportion, however, may not correlate necessarily with seed bank density. In this case, researchers are advised to attempt a correlation between seed bank densities and weed densities at times a + b, a + b + c, b + c, and so forth. Only after these types of assessments have been made can researchers conclude that relationships exist or do not exist between seed bank densities and aboveground vegetation.
The horizontal distribution of seeds across soil determines, in part, how many soil samples need to be taken. Weed seeds typically are not distributed randomly across a field. If they were, sampling seed banks would be much easier. Instead, weed seed banks almost always are highly aggregated in agricultural fields (Wiles and Schweizer, 1999, Chauvel et al. 1989). The aggregation can be the result of very limited dispersal away from parental plants, such as with early-maturing weeds in late-maturing crops (e.g. Avena fatua L. in soybean); or human-mediated dispersal of weeds that mature synchronously with crops, wherein seeds are spread in strips across fields by combine harvesters (e.g. A. fatua in wheat). Such aggregation affects the results of soil seed bank sampling.
The hand-separation and counting processes generally are performed on samples that have dried after sieving and flotation. Unfortunately, seeds of some species (e.g. Impatiens spp.) lose viability quickly after drying, which introduces error in estimates of viable seed densities.
Forcella, F., Buhler, D.D. & McGiffen, M.E. 1994. Pest management and crop residues. pp. 173-189. In Hatfield, J.L. & Stewart, B.A., eds. Crops Residue Management . Lewis Publishers, Ann Arbor, Michigan, USA.
The viability of seeds can be determined through the well-known tetrazolium chloride (TZ) test. The TZ test is simple for the purposes of most weed researchers, but for seed technologists the test can be quite complex. Typically, seeds are soaked in a 0.1-0.2 percent TZ solution for a few hours to one week at 10-30 C, depending upon species and research objectives. The hydrogen released by dehydrogenase reactions in living tissues combines with TZ to form a red pigment. Thus, if seeds exposed to TZ eventually turn pink or red, they contain living tissue, whereas those without the red stain are presumed to be dead.
Arrangement of quadrats probably is not too important provided that the placement of quadrats spans the length and width of the plot or field (Colbach et al. 2000). Some authors place quadrats over or adjacent to the point where soil cores were taken. Although logical, this may have little practical effect on the results given the extreme aggregation of many seed banks.
Our experience is that 5 cm diameter cores represent a practical solution to the problem of core sizes. This size core is large enough to detect seeds, but small enough not to burden the researcher with too much soil. We advocate their use in seed bank studies.