How to Plant & Grow Purple Loosestrife (Lythrum salicaria)


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Purple loosestrife plants by water
Purple loosestrife can usually be found close to freshwater areas, such as riverbanks, marshlands, bogs, and canals. W. Bulach, CC BY-SA 4.0, via Wikimedia Commons

Lythrum salicaria is a hardy, herbaceous perennial native to Eurasia and the British Isles. Most commonly named purple loosestrife, L. salicaria is also known colloquially as long purples, purple grass, rainbow weed, rose loosestrife, sage willow, spiked loosestrife, and willow weed. It belongs to the Lythraceae family of annual and perennial herbs, shrubs, and trees which are generally found near water.

The genus name Lythrum is derived from the Greek Lythron for blood, either for the seemingly haemostatic properties of the plants or the deep, often reddish, shades of their flowers. The species name salicaria, derived from the Latin Salix, meaning ‘willow-like’, is inspired by the shape of the leaves. As the common name implies, loosestrife was believed to remove strife and potential disputes between cattle herds during the time of Alexander the Great. 

In its native Eurasia, purple loosestrife can be found growing abundantly around areas of freshwater, such as riverbanks, marshlands, canals, lakes, and bogs. In gardens, it is best cultivated in moist, sunny borders, or on the periphery of ponds. As a vibrant ornamental wildflower, L. salicaria’s low-maintenance beauty makes a fantastic addition to cottage-style, informal, and naturalistic gardens, whilst its towering, spiked blooms add structure and drama to wildflower gardens, water gardens, and meadows.

Purple loosestrife, as the name suggests, is identified by a dense fuchsia-hued spectrum of vibrant flowers reaching upwards in a spire-like manner, which endure throughout the summer months of June to August. The hermaphroditic flowers are each comprised of around six somewhat irregular petals which emerge upwards from the stem in elongated, spiky racemes, which reach between 10 – 40 cm tall.

Each flower possesses stamen of variable lengths and styles to encourage cross-pollination rather than self-pollination. Its deciduous foliage comprises willowy green leaves which grow in oppositional pairs around the woody, square-shaped, hair-covered stems, from spring through to late summer. Mature plants can reach anywhere up to two meters in height, but average around one-and-a-half meters. It may grow as a single stalk but is more commonly found in dense clumps with a spread of anywhere up to 2 meters.

Facts, Benefits & Uses of Purple Loosestrife

Shrill carder bee on purple loosestrife flower
Purple loosestrife attracts many pollinators, including bees. Ivar Leidus, CC BY-SA 4.0, via Wikimedia Commons

L. salicaria is happiest in wet ground and poorly draining or clay soil, although it can withstand various pH levels, from mildly acidic to mildly alkaline, and can adjust to sandy, loamy, and clay soil conditions. It can also be submerged in low levels of water, thriving as a marginal plant due to the stem tissue which allows the plant to respire. Its impressive adaptability means that it can survive in most light levels, although it will grow most vigorously in full sun. Both versatile and robust, L. salicaria can survive for over 20 years if conditions are right.

L. salicaria offers an excellent source of nutrition for many long-tongued insects in the form of its pollen and nectar. It is pollinated by bees, butterflies, and even moths, and it is often visited by dragonflies and damselflies in the summer months. Because of its long flowering period, it provides food even through the ‘June gap’ period when nectar sources are generally scarce in the UK. If they are not removed, the seed heads continue to attract insects even over the winter months, so this plant is a perfect choice if you want to encourage more native insects and wildlife into your pond garden throughout the seasons.

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Purple Loosestrife Fact Sheet:
PLANT TYPE
Herbaceous perennial
HARDINESS ZONES
USDA 4 – 9, UK zones H3 – H7
LIGHT REQUIREMENTS
Full sun to partial shade
BLOOM COLOR
Purple, fuchsia
BLOOM PERIOD
Summer
MAXIMUM GROWTH
6 feet (1.8 m)
PLANTING DEPTH
Surface sow, do not cover
WATER QUALITY
Freshwater pH 6 – 8

How to Plant Purple Loosestrife

Purple loosestrife in bloom by a river
Purple loosestrife can grow up to a meter tall in their first year of growth, but require 2 to 5 years to reach full height. H. Zell, CC BY-SA 3.0, via Wikimedia Commons

It is best planted in the spring and fall months, although its hardiness means it can survive being planted in mild periods in winter. L. salicaria germinates best in wet soil with relatively warm temperatures and even germinates fully submerged in shallow water.

If you intend to grow L. salicaria from seeds, they can be sown in trays of compost in spring and fall and should be kept moist. Seeds can also be planted directly into borders, but again, must be well-watered. If you are growing from the seeds of an existing plant, they can be collected and sown immediately when they are brown and ripened, around late August to September. If you are allowing to self-seed from an existing plant, the young seedlings should be transplanted to their intended position once leaves begin to appear.

Should you choose to submerge your purple loosestrife at the periphery of a pond, a mesh basket planter is advised for ease of future maintenance, where division or removal may be necessary to control its growth. Purple loosestrife seedlings are particularly fast-growing. They can reach up to a meter in height and may even bloom within the first growing year, although it takes between two to five years for L. salicaria to reach full height.

Propagating from established L. salicaria is fairly straightforward. It should be divided by digging up and separating the roots in periods when the plant is dormant, ideally in fall or spring. If you plan to propagate from cuttings, these should be taken from the new growth in late spring, placed into moist compost, and kept well-watered.


How to Care for Purple Loosestrife

Leaf with leaf spot disease
Purple loosestrife doesn’t tend to have issues with pests, but can be affected by leaf spot disease. If it’s localized, it can be treated by pruning the affected leaves. Forest & Kim Starr, CC BY 3.0 US, via Wikimedia Commons

Although relatively low maintenance, purple loosestrife is a highly competitive, colonizing plant, meaning it can easily suppress and displace other species of both flora and fauna if left to grow unrestricted. Careful planting and maintenance are advised to preserve garden biodiversity and avoid the development of a monoculture, particularly when grown in very wet conditions where the plant is happiest. Light pruning is beneficial both in early spring to encourage new growth, and in early fall to discourage the spread of unwanted seeds. Plants not in ponds should be kept well-watered even when mature.

Purple loosestrife is a particularly voracious self-seeder. The seeds are small, usually less than a millimetre in size. Once matured, a single L. salicaria plant can produce over two million seeds annually and are easily spread by winds, insects, water, and passing animals and humans. The seeds can remain dormant yet highly viable even after long periods in the soil until conditions are right for germination. Additionally, its dense rhizomes can spread aggressively over vast areas once matured.

Each rhizome can produce anywhere between 30 – 50 stems. Over its long lifespan, the base of the plant becomes thick, tough, and fibrous, and these root networks are laborious to fully remove. You might want to consider dividing these clumps every three to five years to avoid overly dense areas. Once established, removal requires digging out rather than just pulling up, and you must take care to remove the whole root network to avoid regrowth. Removal of unwanted L. salicaria plants should be done prior to self-seeding in the fall. Similarly, faded flowers should be removed promptly to prevent rampant self-seeding.

L. salicaria is relatively unperturbed by pests other than slugs and snails, although it is susceptible to leaf spot disease. This can be treated by pruning affected leaves if the affliction is localized. If the whole plant is affected, however, it may need to be fully removed and disposed of carefully to avoid contaminating surrounding plants.


How to Winter Purple Loosestrife

For a short period in the fall, prior to falling at the first frost, the leaves will generally take a deep red or orange shade. Although it loses all of its foliage through the winter, the dead stems will remain until the next growing season and add both structure and interest through the desolate winter months. This hardy plant can withstand temperatures of -25 °C, so little attention is required for overwintering, even for those which are partly submerged in ponds, where it may ice-over.


Is Purple Loosestrife Invasive or Toxic?

The pretty blooms of purple loosestrife are not known to be toxic and are a welcome sight in the summer months in its native Eurasia. In the USA, however, the plant is considered highly invasive. It is thought to have been introduced in the 18th century by European sailors who used its seeds as ballast, or who brought it with them for its medicinal properties. It has no natural predators in the USA, and this, coupled with its extremely competitive nature, means its unrestricted spread has caused severe destruction to the biodiversity of native flora and fauna. Relatively aggressive control measures are employed in some states to prevent its spread, usually by introducing beetles as a biological weedkiller.


Is Purple Loosestrife Edible? Do Animals Eat it?

Both the flowers and leaves of L. salicaria have been used in folk remedies for a variety of ailments, particularly minor stomach problems and bleeding. The vivid flowers have been used to make both hair dye and edible food colouring. There are no known toxic effects to either animals or humans when the leaves or flowers are consumed sparingly, although little is known about the consumption of either the roots or stems. Most grazing animals and birds, however, are unlikely to eat purple loosestrife, possibly due to its high tannin content.


Where to Buy Purple Loosestrife & Seeds? (UK & US)

Both seeds and mature plants can be purchased relatively easily from most garden centers, nurseries, and online retailers within the UK. In some parts of the USA, it is illegal to purchase or cultivate L. salicaria due to its invasive status.

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