Lythrum salicaria Seeds | Purple Loosestrife
LATIN NAME: Lythrum salicaria
COMMON NAME: Purple Loosestrife
Plant Profile
The three floral morphs are called long, mid, and short styled, and each plant produces only one of them, with stamens positioned to match the partner’s stigma. Darwin showed that legitimate crosses between matching levels set far more seed than selfing, and the mechanism is still taught as one of the clearest examples of heterostyly in nature.
Vegetatively the plant is unmistakable once flowering stops: the stems are sharply four angled, the leaves are narrow and willow like, set opposite or in loose whorls, and the whole clump rises as an upright perennial to well over a metre in rich marsh soil. The magenta flower spike opens from the bottom upward over many weeks.
In North America the species escaped gardens and invaded wetlands, forming solid stands that crowd out native cattail and sedge. The principal biological control is a small leaf beetle, Galerucella calmariensis, released in huge numbers so that its larvae skeletonise the leaves and starve the roots of sunlight. Two weevils, a root feeder and a seed feeder, are used alongside it.
European herbals listed purple loosestrife as a styptic for wounds and a gargle for sore throats, uses that track directly to its high tannin load. Modern gardeners are better served by the sterile cultivars bred specifically to flower without setting the aggressive seed that wild plants produce.
Cultivars such as the Morden series and Dropmore Purple are triploid or otherwise seedless selections, giving the same magenta show without the wetland takeover. They are the responsible choice wherever the wild plant is listed as a prohibited invasive.
In regions where the species is invasive, grow only the certified sterile cultivars and never move root fragments to a natural waterway. A single discarded clump can start a stand that takes years and costly beetles to bring back under control.
Science & Biotechnology
The plant’s place in botanical history is secure because it was one of the first species in which the genetic control of flower form was worked out in detail, and students still use its three morphs to see how a population avoids mating with itself.
The species also shows a neat lesson in plant numbers: because the three flower forms must cross to set full seed, a single isolated plant sets far less than a mixed population, so roadside colonies with all three forms outproduce lone garden specimens.
The intense flower colour comes from delphinidin based anthocyanins, the same broad class that paints blueberries and grapes, and the plant is correspondingly rich in tannins. Those tannins are the reason the herb was once valued as an astringent, and they also make the foliage unpalatable to most grazing animals.
Beyond the garden, the dried flower heads hold their colour and are used in floral work, while the astringent roots were once dug for tanning leather because of the high tannin load. Both uses lean on the same chemistry that makes the live plant unpalatable to grazers.
Uses & Benefits
Because the bloom lasts for weeks and produces abundant pollen, the spike is a reliable nectar source for bumblebees and honeybees in mid summer, which is exactly why it was planted as an ornamental before its weedy side was understood.
Purple loosestrife is a plant to admire with a plan rather than a plant to set and forget. If you want the pollinator magnet without the risk, pair a sterile cultivar with other long blooming perennials such as Echinacea purpurea, blue vervain, and scarlet sage. Bulk seed for the pollinator border is listed by the category through commercial growers who supply the group by weight.
Growing from Seed
For the seed itself, note that wild collected seed from invaded wetlands may carry the genes of aggressive clones, so gardeners outside the native range should buy only named sterile cultivars. Sow thinly and prick out early, because the seedlings are slender and easily lost among weeds.
The plant wants its feet wet and its head in the sun: a bog pocket, the edge of a pond, or a container that is never allowed to dry out. Sow the fine seed on the surface of moist soil and keep it bright; the seedlings are slender at first and easy to mistake for grass.
Our Take
A useful comparison for newcomers is that loosestrife in bloom looks like a tall purple spike of garden sage, but the square stem and whorled leaves separate it at a glance. Keep it out of any natural waterway and it is a handsome, long flowering border plant.
Related Reading
Gardeners who grow Purple Loosestrife often pair it with these related subjects: