Invasive plants are a worldwide problem. In the U.S. alone, their economic impact totals billions of dollars every year. It shouldn’t come as a surprise that some of these aggressive species are important nectar and pollen plants, including purple loosestrife, and tamarisk.
Purple Loosestrife (Lythrum salicaria)
Dubbed by one gardening magazine as ‘pretty poison,’ purple loosestrife has found a home in every state except Florida. Most commonly seen in the Northeast and Midwest, it is also present throughout the Pacific Northwest as well.
Purple loosestrife thrives in all climates in partial shade and full sun. Adapted to damp places, and moist soils, this often inhabits ditches. Generally, a wetland species, it is most common along streams, rivers, ponds, lakes, and canals.
Status as a Bee Plant
For years, purple loosestrife has been recognized as an excellent bee plant. Throughout daylight hours in all kinds of weather, the bees work the flowers for nectar and pollen.
Purple loosestrife can yield a surplus of good quality honey. This is often used by the bees as a Winter food source. With yellow combs, this honey comes in a range of colors from light to dark sometimes with a greenish tinge. The flavor varies as well from mild and pleasing to strong. The taste can intensify as the crop gets older.
Also known as spiked loosestrife and purple lythrum, purple loosestrife can reach 1½ to around eight feet in height. This forms a huge clump with 30 to 50 stems. These dense, upright plants grow from woody, thick roots. A mature root can be over 1½ feet long. Hairs cover the stems and leaves. Becoming woody with age, the square to six-angled stems produce numerous branches.
The stalkless, willow-like leaves grow from about four to six inches in length. Quite narrow, they’re only one-half inch wide. Sometimes clasping the stem, these are either opposite or in whorls of three or four.
Purple loosestrife blooms over a long period from June through September. These flowers open on leafy, stiff, erect, spike-like racemes up to three feet in height. Very brightly colored when in bloom, purple loosestrife is considered a beautiful flowering plant. The blossoms range in color from magenta to rose or even pinkish-purple. Shaped like propellers, these flowers are almost an inch wide. They have seven petals. The lemon-green stamens come in several lengths with the shorter ones producing the smallest pollen grains.
Originally from Europe and Asia, purple loosestrife was introduced to the U.S. during the 1800’s by several means. The seeds arrived accidentally in ships’ ballast, and were entangled in the fur of sheep. European immigrants who used purple loosestrife as a medicinal plant brought seeds for their gardens. As a popular flowering perennial, this was grown in home landscapes and parks as well as on golf courses. Despite the fact that its invasive nature was recognized as early as the 1830s, it was sold by nurseries until the 1990s.
How Purple Loosestrife Spreads
This highly aggressive plant spreads by both seed and vegetative means. Even the cultivars that were once considered to be sterile are in fact quite fertile. With so many flower stalks, a plant can produce several million seeds every single year. These are easily carried to new areas by wind, water, and animals as well as by farm and construction equipment, and muddy boots/shoes. Remaining viable for five years, the seeds have a germination rate of 50 to 100%.
The plant’s underground stems grow about a foot a year and enable purple loosestrife to easily spread. Pieces of the plant can be transported to new sites during construction, ditch clearing, and similar activities.
Over 25 states have classified purple loosestrife and all its cultivars as a noxious weed and/or invasive species. As a result, these regulations prohibit selling, growing, distributing, propagating, buying, or moving the plants.
The Impact of Purple Loosestrife
Purple loosestrife poses a serious threat to aquatic habitats by crowding out the native plant species. As a result, wildlife habitats can no longer provide the shelter, food sources, and nesting sites that are needed. Migratory birds have suffered the greatest loss of habitat.
This perennial also serves as the alternate host for a serious plant disease known as the cucumber mosaic virus.
Control of Purple Loosestrife
Prevention works best. Identify the plants early enough so they don’t have a chance to get established.
Control involves physical removal and herbicide treatment. Because digging is so time-consuming, it is only practical when a small number of plants are present. If possible, try to dig them during the early Spring before they get a chance to set seed. After digging, dispose of the plant debris in a hot compost pile or with your regular garbage on trash pick-up day. Once the plants are dug, follow up and monitor the site from time to time for new sprouts. These can be treated with herbicide or pulled.
When purple loosestrife has overtaken a large area, herbicides offer the best solution. The chemical must be one that is labeled for use on aquatic habitats. As with physical removal, it is recommended that you monitor for new sprouts and seedlings after the initial treatment. Re-apply the herbicide if necessary.
Biological control for purple loosestrife seems to be working very well thus far. Two leaf-eating beetles have been released on an on-going basis since the 1990s. Originally native to Europe, these insects can take up to five years to get established. At the present time, they have reduced plant growth at least 70% in most of the release sites.
A species of weevil, a type of beetle, has also been used since the 1990s with some success. In addition, one species of aphid has provided some control in certain areas of the country where it occurs naturally. Though this aphid feeds on the plant, researchers don’t expect this to be as much help as the other beneficial insects. It requires an alternate host plant that is not widely cultivated in the U.S.
Tamarisk (Tamarix spp.)
In the U.S., about 10 species of tamarisk have naturalized with the most notorious being the five-stamen tamarisk (Tamarix ramosissima).
Native to Europe and Asia, these were introduced to the U.S. during the 1830s for wind breaks, erosion control, and ornamentals. Within just a few decades, they escaped in the Southwest. Now, they are found wild in 45 or so states. Yet, most damage occurs in the West where these woody plants occupy over 1½ million acres.
Growing to an elevation of around 7500 feet, tamarisk prefers sunny sites with moist soils along waterways, such as streams, rivers, springs, lakes, irrigation ditches, canals, and seasonally flooded sites.
This woody plant is adapted to salty, alkaline soils. The hardiest species are the five-stamen tamarisk, and the small-flowered tamarisk (Tamarix parviflora).
Tamarisk’s Status as a Bee Plant
In the West, tamarisk is an important honey and pollen plant. The fact that this blooms for such long periods is very helpful to bees.
The plant often yields a good surplus of honey, around 100 pounds per colony. This tends to be dark-colored, usually a dark amber. Somewhat on the strong side, the flavor is reminiscent of horehound. Since this crop can be less than desirable, it isn’t usually mixed with better quality honeys.
Tamarisk can assume the form of a large shrub or a small, shrub-like tree with multiple trunks. Depending on the species, it can be deciduous or evergreen. The height varies considerably from five feet to 20 feet or more.
This woody plant has a fine texture due to the small foliage, the thin stems, and the slender, numerous branches. The reddish-brown bark develops ridges and furrows as it gets older. Delicate and scale-like, the foliage is gray-green to pale green.
Very floriferous, tamarisk blooms for an extensive period from Spring until Autumn, producing literally thousands of white or pink blossoms. These have five petals. Generally, the heaviest flush of flowers will be in the Spring season.
How Tamarisk Spreads
The long bloom season allows a single tamarisk plant to produce over half a million seeds annually. A tuft of hairs on the seeds aids their spread by wind and water. Animals can also carry these to new locations. The earliest crops of seeds have a nearly perfect germination rate. Viable only for about two months, they germinate within 24 hours.
Tamarisk also spreads vegetatively by means of its roots, crown, and stems.
Legal Status of Tamarisk
Also known as salt cedar, tamarisk is officially considered a noxious weed in 45 states, particularly in the West. In addition, it is also classified as an invasive plant in some locales. These regulations make it illegal to sell, distribute, or grow the plants.
The Impact of Tamarisk
Tamarisk is a concern for several reasons. This aggressive plant spreads very fast and grows so quickly (over 10 feet a year) that it overwhelms and displaces native plant species. It diminishes natural diversity and degrades wildlife habitats that ordinarily provide shelter, nesting sites, and food.
Creating a dense thicket, tamarisk establishes a monoculture. These stands pose a serious fire hazard in the West. They also increase the likelihood of flooding along waterways.
With an extensive root system that grows 10 feet below the soil surface, this thirsty plant absorbs all of the moisture it can reach. This causes the water table to drop, leaving native plant species with little moisture.
Tamarisk contributes to long-term soil salinity. This plant exudes salt through its foliage, which becomes deposited on the soil surface when the leaves are shed.
Control of Tamarisk
The best approach is to be proactive. Landowners should never allow it to get established. Monitor all vulnerable areas. Pay careful attention to spots that have been burned or overgrazed, or where the soil has been disturbed. Such sites should be planted with native species that are well-adapted to local growing conditions. With care, these natives can get established before tamarisk makes its presence known.
At a young stage, tamarisk can easily be pulled and destroyed. Once this becomes established and spreads, control becomes harder. Herbicides are often used for tamarisk, sometimes in combination with physical removal. Choose a chemical formulation that is labeled for use on aquatic habitats. As a follow-up, monitor the site for new suckers or sprouts that emerge after the herbicide is applied. If necessary, remove or spray these.
For large areas, root plowing is an excellent way of physically removing tamarisk. This is done to a depth of 1½ feet. In conjunction with the root plow, herbicides can easily be injected into the crowns of the plants.
Other combinations of methods are also effective. One involves cutting the trunks and treating the stumps with herbicide. The shorter the delay between cutting and applying the chemical, the better the result will be. However, this procedure is actually more complicated than it seems. The treated stumps must remain undisturbed until the herbicide has a chance to kill the roots – a process that can take several years. For that reason, avoid burning or flooding the area.
Several forms of biological control work fairly well for tamarisk. One of the simplest is to let cattle graze on the plants. This is a great way to get rid of new sprouts. In some areas of the country, there is a naturally occurring leafhopper that helps to some degree. In California, this species reduced growth in 75% of the cases.
For warmer areas, one particular species of mealy bug has been introduced. Generally, this insect produces several generations a year. A leaf beetle from Asia has also offered some control. This was initially released in some western states in 1999. In its larval and adult stages, it feeds on the Asian tamarisk (Tamarix pentandra) and the small-flowered tamarisk.
Tamarisk and purple loosestrife are only two of the invasive plants that are good bee plants. Next time, we’ll look at some others.