Highly invasive plant species are aggressive and can spread to dominate other species. Invasive plants include not only noxious weeds, but other plants not native to this country. Plants can be invasive if they have been introduced into an environment where they did not evolve.

Legally, a plant is called a noxious weed when designated by a Federal, State or county government as injurious to public health, agriculture, recreation, wildlife or property. They reduce the diversity and value of the habitat and tend to be very difficult to control. However, bees need pollen and nectar from flowers and foraging from noxious weed species is a natural for honey bees and native bees.

Gardening magazines have identified Purple Loosestrife (Lythrum salicaria) as “pretty poison” and it has found a home in every state accept Florida. Purple Loosestrife thrives in all climates, from partial shade to full sun. Bees work the flowers for nectar and pollen throughout daylight hours and often save the quality surplus honey as a winter food source.

Purple Loosestrife is spread by both seed and vegetative means. The plant can produce several million seeds each year that are carried to new areas by wind, water, animals, muddy boots as well as by farm and construction equipment.

Tamarisk or saltcedar (Tamarix spp.) is a honey and pollen plant because it blooms from spring until autumn, producing literally thousands of white or pink blossoms with five petals. This woody plant has a fine texture due to the small foliage, thin stems and slender, numerous branches.

The long bloom season produces over half a million seeds annually. Tamarisk spreads by seeds (600,000 wind-dispersed seeds per year per plant) and can spread upstream quickly. It also secretes salt making the soil too salty for native plants. A single plant can transpire 200 gallons a day - the same amount of water that a small family might otherwise use. Animals also carry seeds to new locations. Growing along waterways, Tamarisk also spreads vegetatively by means of roots, crown and stems. The germination success rate is nearly one hundred percent.

Canada thistle (Cirsium arvense) is regarded as a good source of nectar and pollen for honeybees. It is common to roadsides, railway embankments, lawns, gardens, abandoned fields, agricultural fields, margins of forests and waterways. A rhizomatous plant, with underground propagating roots and stems, forms colonies.

Yellow starthistle (Centaurea solstitialis) blooms mid-summer and thrives in dry, semiarid conditions. Limited populations have been found in Boulder, Larimer and Jefferson counties. It is considered a noxious weed, colonizing and spreading rapidly, displacing vegetation and producing sharp, toxic spines. Ingesting large amounts of the sharp flowers can be fatal for livestock, but bees love the nectar from this prickly plant. Nectar from yellow starthistle flowers produces a unique honey, light in color with a slightly sweet flavor.

Since 1990, the Colorado Department of Agriculture has protected the state’s natural resources by enforcing regulations controlling noxious weeds with the Colorado Noxious Weed Act. How can individuals be stewards of the land and protect forage bees?

According to Colorado State University Extension Fact Sheet on Weed Management for Small Rural Acreages, the most common strategies for controlling noxious weed populations are a combination of two or more methods. Cultural methods promote the growth of desirable plants using fertilization, irrigation and planting at optimum densities so that plants will compete with weeds.

Most gardeners know about using mechanical controls that physically disrupt weed growth. Identifying weeds early and hoeing, tilling, hand-pulling, mowing and burning are examples. Mulching weeds is often considered mechanical even though it excludes light rather than physically disrupting weed growth.

Biological controls use an organism to disrupt weed growth. Often the organism is an insect or disease and natural enemy of the weed. Examples of this would be the bind-weed mites available early in the season for non-irrigated land and the use of sheep and goats to graze on weeds.

Chemical controls use herbicides to disrupt weed growth. Reading the label is the first rule before using the product and following all directions and precautions. Using herbicides on noxious weeds is always part of an integrated control strategy in combination with cultural, biological, and mechanical controls.

Most honey bee poisoning occurs when (a) a toxic substance is applied to a crop in bloom, (b) spray drifts over to flowering plants, (c) bees collect contaminated pollen and nectar, and (d) bees come into contact with chemical residue in water. Beekeepers are responsible for informing cities and counties regarding the location of their beehives.

There is not a simple answer in control of noxious weeds. Kelly Uhing, Colorado State Weed Coordinator, states, “Weed scientists and land managers across Colorado have been working very long and hard to educate the public about the threats noxious weeds pose to the environment.”

The Colorado State noxious weed law may be found at the following web site: http://www.ag.state.co.us/CSD/Weeds/statutes/weedlaw.PDF

PlantTalk www.planttalk.org scripts: #2103 Colorado Noxious Weeds, #2102 Canada Thistle, #1429 Pesticide Safety

National Pesticide Information Center: http://npic.orst.edu/

The authors have received training through Colorado State University Cooperative Extension's Master Gardener program and is a Master Gardener volunteer for Larimer County.

Gardening and Insect Fact Sheets are available on-line by clicking HERE.

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This page updated:  August 27, 2007