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Non-Native Invasive Plants

By Irene Shonle, Colorado State University Cooperative Extension, Horticulture

If your neighbor told you she had a plant that was beautiful, long blooming, drought-tolerant, deer-resistant and spreads very quickly, you might respond that it sounds too good to be true.

In fact, it is too good to be true.

Colorado has a problem with non-native invasive ornamental plants that displace native plants, reduce biological diversity and alter ecosystem processes. Many of these are on the State Noxious Weed List, making them illegal to sell or plant.

The Brooklyn Botanical Gardens estimates that there are 300 dangerously invasive weeds growing in the continental U.S. and Canada and of these, half were introduced as ornamentals. They were brought to this country intentionally and allowed to gain a foothold before their harmful effects were known.

When they arrived in this country, none of the mechanisms that keep plants in check, such as insects, disease and competition came with them. So when they are unmanaged in native areas, they take over and disrupt the ecosystem, affecting bird, insect, fish and mammal populations that depend upon native plants for food, shelter and protection from predators.

And it's not really possible to plant invasive ornamentals responsibly. Seeds can be eaten by birds, carried by cars, dogs or the wind and then may be planted in new locations.

Some of the worst ornamental invaders in Colorado include purple loosestrife, ox-eye daisy, Russian olive, tamarisk, Bouncing Bet, Dame's/sweet rocket, perennial sweet pea, Dalmatian toadflax, yellow toadflax, Mediterranean sage, common tansy, scentless chamomile, and myrtle spurge.

Purple Loosestrife (Lythrum salicaria) is an aggressive weed that is overtaking U.S. wetlands at the rate of 475,000 acres each year.

It thrives in moist soil, near rivers, streams, irrigation canals, drainage ditches, lake shores, wet meadows and marshes. It's easiest to identify when the purple-magenta flowers bloom from mid-June through mid-September.

The blossoms have five to six petals and grow in clusters at the end of long spikes. Each plant is capable of producing 1 to 3 million seeds annually. Some cultivars carry the claim of sterility but recent research has shown that these varieties can and do produce viable seeds.

Plant instead: Spotted gayfeather (Liatris punctata), Russian sage (Perovskia), butterfly bush (Buddleia), delphinium or larkspur, blue vervain, lavender, wild lupine, violet sage (Salvia x superba), and fireweed (Epilobium spp).

Ox-eye daisy (Chrysanthemum leucanthemum or Leucanthemum vulgare), a member of the sunflower family, is an erect perennial plant with white ray and yellow disk flowers that bloom from June through August.

A native of Eurasia, this aggressive plant has escaped cultivation and become a troublesome weed in the mountains. In Crested Butte, this plant is crowding out many of the wildflowers they are famous for.

Plant instead: Native daisies (Erigeron spp.), shasta daisy, black-eyed Susan (Rudbeckia hirta), and blanket flower (Gaillardia aristata), the 1998 Plant Select plant Purple Mountain Sun Daisy (Osteospermum barberiae v. compactum 'Purple Mountain').

Yellow toadflax (Linaria vulgaris) is so widespread that you can often find it mistakenly listed as a native wildflower in field guides, and not as a noxious weed.

It still is sold by some seed companies and nurseries as "butter and eggs" or as "wild snapdragons."

Always look on the back of wildflower seed mixes for a listing of what's included in the mix. If toadflaxes are listed, do not buy that product.

Yellow toadflax is adapted to a variety of conditions, from moist to dry and does well in all types of soils. Because of its early vigorous growth, extensive underground root system, and effective seed dispersal methods, yellow toadflax is difficult to control.

Plant instead: Annual snapdragons, coreopsis, yellow columbine (the 2001 Plant Select variet was Aquilegia chrysantha 'Denver Gold'), 'Silverblade' evening primrose, (Oenothera macrocarpa spp. incana 'Silver Blade'), golden banner (Thermopsis spp.), wallflower (Erysimum Asperum).

Dame's Rocket (Hesperis matronalis) is also known as Dame's Violet or sweet rocket. It tends to invade riparian and wetland habitat, but can also naturalize in other habitats.

It still can be found in wildflower seed mixes, so if you see it listed on the packet, do not buy it.

This European native may be either a biennial or perennial, and may be from 1 1/2 to 4 feet tall, with flowers ranging in color from white to pink to purple. There are four petals, which help to distinguish it from phlox. Dame's Rocket flowers from April through July.

Plant instead: Blue columbine (Aquilegia caerulea), native bee balm (Monarda fistulosa menthaefolia), woodland phlox (Phlox divaricata 'Chattahoochee'), native harebells, (Campanula rotundifolia).

Bouncing Bet: (Saponaria officinalis) can form dense stands of plants, especially in disturbed areas along roadsides.

It grows up to 3 feet tall, with clusters of pink to white flowers on the tops of the plants. Each flower has five petals, all with a distinctive notch at the end. The plant has a strong creeping rootstock and opposite, strap-like leaves. It flowers from July to September.

Planting alternative: Garden phlox, (Phlox paniculata), 'Prairie Jewel' penstemon, Rocky Mountain beeplant (Cleome serrulata), native yarrow (Achillea lanulosa).

Photo: Judy Sedbrook

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Date last revised: 01/05/2010