By SUSAN PARK

Years ago, when our ancestors came to settle in the Americas, they brought with them some of their favorite seeds. The tradition still continues. When my sister Kathy came back from California, she brought an avocado tree which my mother still keeps alive through winter in her basement. The dogwood my grandmother attempted to transplant from North Carolina to her home in Clearwater, Fla., didn't do as well.

Unfortunately, not all plants should be moved from their native habitat to a new one. The results can be disastrous. Mother Nature had good reasons for keeping Scot's Broom in Europe where it was kept in check. The millions of plants we see each spring come from only three original plants that were introduced in 1850 by Captain Walter Colquhoun Grant who settled in Vancouver, British Columbia.

Likewise, blackberries, although great for picking, have taken over our Seattle yards because they flourish in our heavy rainfall and mild climate. Hard to imagine that on the other side of the country, blackberries are kept in check by the hot summer droughts and cold winter ice storms. Also on the East Coast: Kudzu, a thick lush vine from Africa introduced to help keep slopes from eroding, now covers almost everything from houses to tall trees. Removal is achieved only through bulldozing.

It's hard to resist the temptation of taking your favorite plants with you when you move.

ONE PARTICULARLY attractive flowering shrub, purple Loosestrife, was also introduced sometime during the settlement of Eurasian farmers in Puget Sound. The name Loosestrife, meaning "deliver from strife," came from its ability to calm farm animals. It also works as a mild insect repellent.

Loosestrife, although beautiful, can completely take over marshes and the edge of lakes, crowding out all native plants. Because of its beauty, unknowing gardeners may pick the flowers and carry the seeds home to plant in their own yards. A heavy infestation at Foster Island near the Arboretum proves irresistible to Sunday strollers.

Similar to Fireweed, Loosestrife grows between 18 inches and six feet in tall spikes. Its purplish pink flowers are different in that Fireweed has four petals and Loosestrife has five. Loosestrife is identifiable from its square stems. Fireweed, a member of the Primrose family, has gentle lily-like leaves with visible veins on the underside that are attached to a round, hollow stem. Loosestrife prefers to grow in marshy areas beside creeks and lakes while Fireweed prefers drier ground.

FIREWEED, A NATIVE to Puget Sound, has played a useful part for many centuries in Native American cultures. Its stems were peeled and spun into twine. Its fluffy seeds which fly around each fall were used to stuff mattresses or woven into blankets. Its leaves are a healthy source of vitamin C. Loosestrife, on the other hand, is not eaten or utilized by any animals or humans while it crowds out native plants that are beneficial to wildlife. Each mature plant produces about a million seeds a year.

The State of Washington has officially designated Loosestrife as a "noxious weed," meaning that it is under quarantine. It is illegal for anyone to plant Loosestrife. Landowners are required by law to control or remove it.

King County is currently working to eradicate the plant by forming volunteer work parties to dig it up. Because of its proximity to water and wildlife, chemicals should not be used to control it. Instead, dig it up from the roots and place them in a sealed plastic, trash bag. The bag should be disposed of in the regular garbage rather than being mixed with composting. If the plant has already gone to seed, carefully cut off the head and put it in a bag before trying to remove the plant.

If you would like to volunteer to help remove Loosestrife, or have questions on how to identify it, call the King County Noxious Weed Board at 296-0296.