For 30 years they lie beneath the soil waiting for the aliens to leave. But if it takes any longer than that for the aliens to disappear, they die and will be lost forever.
It may sound like the premise for a Stephen King novel, but it's actually about one of the Western Cape's most prized natural possessions - the fynbos.
The restoration of this much-loved vegetation is possible in areas even where pines have taken over for three decades because the little seeds are still in the soil.
After that, they begin to die off, so the advice from researchers at the University of Stellenbosch is this: To save costs, time and effort when rehabilitating sites, pine plantations and areas invaded by pines should be cleared preferably before 30 years are up, and certainly before 40 to 50 years have passed.
"Pine plantations and pine invasions have numerous impacts on native ecosystems in the fynbos biome of South Africa," said lead researcher Alistair Galloway, of the department of conservation ecology and entomology at Stellenbosch.
The extent to which the ecosystem could recover was "determined by the severity" of the impact of the pine plantations.
Galloway's work has just been published in the South African Journal of Botany.
"The longer you wait, the less likely the chances that any fynbos seeds will be left in the soil to sprout successfully," he said.
This research is important because of the central role pines have played in the botanical land-grabs in the Mother City.
Attempts to remove pine plantations have been very active in the past few years, while attempts to restore fynbos are equally as vigorous.
Newlands Forest, for example, was once a purely indigenous forest where Khoi people lived, but then, about 200 years ago, it became clear that commercial logging could be a lucrative business if pines were planted.
This happened in many areas, and led to the proliferation of many different alien species that were brought in by the human hand and soon took over vast tracts of land.
SA National Parks lists pine and other alien species as "one of the biggest threats to the biodiversity of the Table Mountain National Park".
Pine finds itself on a list of alien invasive species alongside Port Jackson, rooikrans, wattle, hakea and blue gum.
According to SANParks, pine trees and other invaders "destabilise river banks" and "use up precious water supplies".