CPSC 226: Introduction to Weed Science

Soil Seed Banks

To help you gain an appreciation for soil seed banks, I have put together a few graphs to illustrate how rapidly seed banks build up, and how long it would take for seed bank drawdown to occur.

Let's start out by looking at seed bank drawdown. Pretend that you have the ambitious desire to remove all of the seed from your one-acre field. Furthermore, let's start out with the following assumptions:

1. There are initially one million viable seeds in your field
2. Each year half of all the seeds present germinate (the other half remains viable in the soil)
3. Using a combination of herbicides and cultivation, you kill 99.5% of all the weeds that emerge
4. Each weed that escapes control produces 1000 seeds, but fortunately, only 10% (100) of those survive (the rest fall victim to field mice, sparrows, and fungi, your biological control helpers!)

• Given this scenario, how long before there are no longer any seeds in your field? Click here.
• Okay.. so you are going to have to be even more ambitious than you thought, and do some serious weed management. You supplement your herbicide and cultivation with some hand hoeing, so now you kill 99.9% of the emerged weeds. The results.
• Alright, you decide to try one more trick. In the spring you perform a couple light tillage operations to induce more of your weed seeds to germinate. So now let's assume you have 80% seed germination and, since you still include hand-hoeing, you kill 99.9% of all the germinating weeds. Now have you achieved rapid seed bank drawdown?
• A more realistic scenario might be one in which you control 99% of the germinating weeds. Then, if you assume that each survivor produces 100 seeds, regardless of what percentage of the seeds in the seed bank germinate, this will be the result.
• So you can see, in most cases, complete drawdown of the soil seed bank is not a reasonable goal. To further illustrate this point, let's assume that you did succeed in drawing down the seed bank to just a few individuals. What happens if you become lax on your weed control for just a few years?
  
  Specifically, let's assume that the starting population is 100 seeds, 50% germinate each year, you control 90%, and those not controlled produce 100 seeds each. This is how quickly the seed bank would build up.

So even if, after several years of intensive weed management, you were able to draw down the soil seed bank, you would still need to keep up that intensive management if you wanted to keep the seed bank down.

Now, if only I can get my bank account to behave more like a seed bank: slow to draw down and quick to build up!

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