Seed bank

This article is about the reserve of seeds in the soil. For financial institutions, see Bank. For facilities that store seed deliberately, see Seed vault.

A soil seed bank is the natural reserve of living seeds in the soil of an area. Seeds that do not germinate at once may survive for months or years, so that a plant population consists not only of the plants that can be seen but of a much larger number that cannot. The seed bank replaces plants that die or are removed, and allows a population to recover from disturbance.¹

For an invasive plant, the seed bank rather than the visible stand usually determines how long control must last. Giant hogweed is a clear case: its plants are easily found and killed, but its seeds, lying invisibly in the upper layer of the soil, go on producing new plants for years after the last parent has gone.²

Dynamics

A seed bank has one input and several outputs. The input is the seed rain: the seed shed onto an area each year. The outputs are germination, which turns seed into plants; death from age, decay and fungi; and predation by animals. Where the input is large and the outputs slow, the reserve accumulates from one year to the next.

In this sense the seed bank functions as the population’s reserve army: at any moment most of its members are out of sight, waiting for a place in the stand to become free. Removing the plants without touching the reserve simply frees those places.the reserve army of seeds

In giant hogweed

year  viable seed per m²
   0  ████████████████████████████████████████  3,760
   1  ███▌                                      331
   2  █                                         102
   3  ▍                                         45
   4  ▎                                         24
   5  ▏                                         13
   6                                            7.1
   7                                            3.8
   8                                            2.0
   9                                            1.1
  10                                            0.60

Giant hogweed sheds on average some 20,000 seeds per plant,³ which fall mostly within a few metres of the parent. Almost the whole seed bank — 95 per cent — lies in the top 5 cm of the soil. Under established stands the density of living seed was found to fall from about 3,760 per square metre in autumn, just after seed fall, to 192 per square metre by the following summer, as most seeds germinated or died.¹

What remains declines steadily. In seed buried at ten localities across the Czech Republic, 8.8 per cent was still viable after one year, 2.7 per cent after two and 1.2 per cent after three;⁴ after five years 0.4 per cent, and after seven 0.1 per cent.² The tendency of the viable fraction to fall is well established; its complete exhaustion is rarely observed. The last fraction is the part that matters. Even 0.1 per cent of the 20,000 seeds of a single plant is some twenty seeds, enough to start the invasion again.²the 1% is enough

The fifteen-year myth

Many guides state that hogweed seed can survive in the soil for fifteen years. The figure goes back to a Swedish conference paper of 1989, which said only that some data suggested germinability “could extend up to fifteen years”, without saying whether the seed had been buried or stored dry. Passed from one review to the next, the suggestion hardened into a fact; the researchers who traced it called it an urban myth.⁵ Seed stored dry in fact loses almost all of its germinability within about three years.⁴

The practical conclusion is unchanged, however. A site must be kept free of seeding plants for at least five years after the last plant is removed, and watched for longer.⁶²

Depletion

A seed bank can be exhausted only by stopping all new input for several years in succession, while the seedlings that continue to emerge are destroyed before they can flower. Any interruption — a single plant allowed to set seed — restores the reserve to the level of several years earlier, as the figure shows. Programmes are therefore judged not by the number of plants removed, but by the number of years without seeding.or open it yourself: ↑ ↑ ↓ ↓ ← → ← → B A

See also

• Giant hogweed
• Control of giant hogweed
• Dormancy and stratification
• Seed rain