Chapter 4: Seed Reproduction
4.3 Weed Seed Bank (also known as soil seed bank)
The weed seed bank is a description of the number of and types of viable seeds present in the top eight inches of the soil. This is the depth at where our normal crop production is going to be taking place. Each field will have its own unique and ever changing seed bank.
Viable seed is alive and respiring, with a healthy baby plant – embryo. Under the right environmental conditions, it will germinate and produce a seedling.
In contrast….
Dormant seed is also alive and viable, but it will not germinate even under favorable crop conditions (ie temperature, soil moisture, light). There is some other factor that is keeping it from germinating. We will discuss dormancy in more detail in the next section. For now, just understand that the seed bank also includes dormant seed.
Some interesting facts to keep in mind regarding the seed bank:
- Only about 5 to 10% of the viable seeds germinate per year under good environmental conditions.
- Nearly 95% of viable seeds will decline (lose ability to germinate) in the soil in 5 years. Therefore, if weed plants are not allowed to reproduce for 5 years, the weed seed bank numbers will be significantly lower. However…
- Some seeds remain viable even after 100 years in the soil.
- Some weed plants can produce tens of thousands of seeds (or more) per plant! Compounding this issue, if you have put high selection pressure on the weeds in your control strategy, many of those seeds could be resistant or are quickly becoming resistant to your management tools.
Practice Activity
To help you put the amount of seeds weeds can produce into perspective at the field level, match the plant below with the normal plant population seeding rates per acre.
Explanation: If only 5% of a weed seed bank germinates in a given year, an acre of farmland may have over 100 million weed seeds already in the soil, which means 5 million weed plants/acre may be present. If you have been applying strong weed control methods, you have have placed high selective pressures on the populations, so a great amount of those weed seeds will be resistant or troublesome.
This table lists some common weeds and the number of seeds an individual plant is capable of producing.
| Weed | Number of Seeds/Plant |
| Yellow Nutsedge (Cyperus esculentus L.) | 2,400 |
| Pennsylvania Smartweed (Polygonum pensylvanicum L.) | 3,000 |
| Barnyardgrass (Echinochloa crus-galli (L.) Beauv) | 7,000 |
| Giant Foxtail (Setaria faberi Herrm.) | 10,000 |
| Common Ragweed (Ambrosia artemisiifolia L.) | 15,000 |
| Velvetleaf (Abutilon theophrasti Medik) | 17,000 |
| Jimsonweed (Datura stramonium L.) | 23,400 |
| Shepherd’s Purse (Capsella bursa-pastoris (L.) Medik.) | 38,500 |
| Curly Dock (Rumex crispus L.) | 40,000 |
| Common purslane (Portulaca oleracea L.) | 52,000 |
| Common Lambsquarters (Chenopodium album L) | 72,000 |
| Redroot Pigweed (Amaranthus retroflexus L.) | 117,00 |
| Russian Thistle (Salsola tragus) | 200,000 |
| Eastern Black Nightshade (Solanum nigrum L.) | 800,000 |
Before concluding this section, it is important to discuss the different factors affecting the number of weed seeds present in the seed bank. Past management strategies will play a large role in the state of the seed bank. Take for example tillage and think back to the idea we discussed earlier about ecological succession. If you are tilling under the soil each year, you are creating an environment that favors annual weeds, whereas reduced tillage practices will encourage more perennial weeds over time.
The number and types of weeds seeds added to the seed bank also depends on your crop rotation, with different crops better able to compete with different weeds.
Similarly, if you incorporate herbicide treatments, it will affect the seed bank composition. There may be fewer weed seeds present (and fewer types of weeds), but a greater percentage of them are likely to be resistant to the particular herbicides routinely used.