by Julia Parry - Colorado State University Cooperative Extension Master Gardener, Denver County
Several months after I bought a new home in January 1989 - one of Denver's coldest recorded months - it was a pleasure to see that plants in my new garden had survived the winter and begun to grow.
Later, the Denver Botanic Gardens Herbarium identified samples from my yard. The results? Fifteen of sixteen "plants"were weeds.
Though disappointed, I thought how bad can a weed be anyway? For some weeds, I've learned the answer: Very bad.
Certain non-native species can spread with alarming tenacity and speed. They may preempt native Colorado plants when they escape into the wild.
A prime example is purple loosestrife. Seductive as a call girl in a saloon, this mid-summer blooming spiked beauty grows to five feet along stream banks. Its seeds spread downstream where it takes over more and more stream banks, displacing cattails and other vegetation used by wildlife. Unfortunately, ducks and other wild animals find the plant invader useless.
Purple loosestrife is a concern along Bear Creek, Clear Creek and other drainages in the west Denver metro area. The state Division of Wildlife is attempting an eradication effort. Some horticultural varieties of loosestrife grown in local gardens are sterile as far as seed production, but add to the weed problem by producing viable pollen which can pollinate the flowers of stream bank runaways. One plant produces several million seeds per year.
Many less aggressive, spiked flower species are available to fill the same niche in the garden. Gardeners concerned about environmental problems with loosestrife should consider lupine, penstemon, veronica and other species of the same color and shape that don't invade our waterways.
Loosestrife is not the only aggressive invader. With no natural insect enemies, myrtle spurge has escaped from Golden and Genessee gardens and now covers some hillsides. Dalmatian toadflax and butter and eggs toadflax also are aggressive, but arrive daily in wildflower seed packets. Oriental clematis is now spreading in the Clear Creek drainage. Other tamer groundcovers and clematis varieties are available.
What can concerned gardeners do? First, find out what plants are so aggressive that they become weeds. Next, ask what plants can give you the same flower color, shape and plant growth habit. Purchase those instead of plants that will become a nuisance. Finally, support efforts to slow the spread of these aggressive plants into native habitats.
Purple loosestrife photograph courtesy of North Dakota State University, Rodney G. Lym, Professor, Plant Sciences Dept., "Identification and control of Purple Loosestrife".
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010