Reprinted from Mt. Lebanon Magazine September 2012
By Ron Block and Deborah Larson
We can walk, drive, or fly to get from place to place, but have you ever considered how a plant can turn up in new territory? The parents don’t move, but the progeny can spread far and wide. A walk this fall in one of Mt. Lebanon’s three passive-use parks-Bird Park, Twin Hills Trails and Robb Hollow-could be a wonderful opportunity to observe the different ways seeds are dispersed.
For many plants, wind power is the method of choice. The seed will be light, and may come equipped with a parachute that allows the wind to carry the seed far away from the parent plant. Since the wind can be fickle and may drop the seed in unfavorable conditions, these plants-milkweed, cottonwood, dandelions, thistle and fireweed, for example- tend to produce an abundance of seeds. Also very effective are the winged seeds of the maple and the ash, which spike rotors and can be carried far from the shade of the parent plant. These seeds will be very dry before they fall, making them light and able to travel good distances.
Some plants produce fruits that are eaten by animals and birds. The fruit is ingested, and the seed, protected by its seed coat, passes unharmed through the digestive tract to be deposited at a distance from the parent plant, with the bonus of added fertilizer. Some such plants are blackberry and black raspberry, pokeweed and cherry, as well as other less-desirable plants such as poison ivy and bittersweet. These often appear in yards at random…. left by a passing bird, stopping to rest on a post or tree.
Squirrels, chipmunks, and other hoarding animals and birds aid by planting seeds. Their efforts to hide edible nuts and seeds for later use, which are then forgotten, can end up creating new trees such as oak and hickory.
Yet another method is for a seed to hitch a ride by attaching to fur, feathers or clothing. Many plants produce burrs which cling to surfaces that come in contact with them. Burrs come in many different shapes and sizes, from the large round burr of burdock, which inspired the invention of Velcro, to the thin double-pointed achene of the bur-marigold, also known as beggar’s-ticks or sticktight. These cling to clothing and will ride with you or your dog back to your yard.
Others have small seed that can lodge in shoes or the paws of animals. The Native Americans called broadleaf plantain “white man’s footprint” , because its ability to thrive in compacted and disturbed soils made it an unmistakable sign that the settlers had arrived. Each plant can produce up to 20,000 seeds, which ensured this non-native a quick spread across the land.
Some plants produce seed pods which burst open explosively and disperse seeds by flinging them far from the parent plant. Jewelweed is such a plant and grows extensively in our parks. Its other name, “touch-me-not”, refers to this ability to send a seed several feet away when mature. Another is hairy bitttercress, which explodes when the ripe seedheads are bumped.
Other plants let gravity do the work, by growing along streambanks. Often these plants grow over the banks, and their small, light seed can float many miles downstream until they lodge in fertile areas, particularly after floods. Willows and birches are two larger plants employing this method, while foxglove is a smaller one. A more problematic plant is the invasive Japanese Knotweed, which can be found crowding watercourses throughout the region.
After a seed has found a favorable territory in which to establish with less competition, it must often then survive for many months until growing conditions are once again favorable. By examining seeds closely one can see the hard protective coat which surrounds the embryo and also the food storage tissue, or endosperm.This three-layer structure helps ensure winter survival.
We encourage you to visit your local parks this fall, and see how many of the methods of seed transport you can discover.