Purple loosestrife

Sun

Full sun is best; tolerates light partial shade with fewer flowers.

Water

Needs consistently moist to wet ground; tolerates waterlogged soil and shallow standing water. Poorly suited to dry borders.

Soil

Moisture-retentive loam or clay, including boggy soil; tolerates a broad range of soil pH.

Height

3-5 ft (0.9-1.5 m)

Spread

1-2 ft (0.3-0.6 m) per clump; seedlings can spread far beyond this.

Flowers

Summer to early autumn; dense spikes of purple-pink to magenta flowers.

Foliage

Narrow, pointed green leaves, usually opposite or in whorls, on upright, often square stems; dies back in winter.

Growth

Fast

Care needed

Medium

• Pollinators
• Cold hardy
• Heat tolerant
• Low allergen

Northern hemisphere months. In Australia, shift them by six.

Difficulty

Moderate

Toughness

Fairly tough

Pick your zone, country, light and whether you have pets.

1. Planting day

   1 ft (0.3 m)

2. After 1 year

   2.5 ft (0.76 m)

3. After 2 years

   4 ft (1.2 m)

   full size

About 2 years to full size.

From a typical nursery size, at an average rate for this kind of plant. A rule of thumb, not a forecast.

Cats and dogs

Safe

Native to

the UK and Australia

Invasive in

the US and Canada

In the garden

Spreads vigorously: plan to contain it.

Growth rate

Fast

• Check local restrictions before obtaining plants; do not plant it in the United States or Canada.
• Where appropriate and legal, plant in reliably damp ground and water during dry spells before the soil dries out.
• Avoid routine feeding; rich soil can produce tall, floppy growth.
• Apply a thin layer of compost in spring if the soil is poor, keeping it clear of emerging shoots.
• Remove unwanted seedlings promptly and deadhead before seed capsules mature.
• Divide congested clumps in spring only where growing and moving the plant is appropriate and legal.

Pruning. Cut flower spikes off as they fade, before seed capsules mature. Cut dead stems close to ground level in late autumn or early spring; bag seed-bearing material rather than putting it in home compost.

Early spring

Cut back any remaining dead stems and check surrounding ground for unwanted seedlings.

Spring

Within its native range, and where legal, plant or divide clumps in damp ground.

Late spring

Check soil moisture and add discreet support if shoots lean in rich soil or shade.

Summer

Water during prolonged dry weather and inspect leaves for feeding damage.

Late summer

Remove fading flower spikes before seeds develop; repeat as later spikes finish.

Autumn

Remove any seed-bearing stems carefully and inspect pond margins or nearby ditches for escapes.

• Within its native range, use beside a garden pond for upright summer colour above low marginal plants.
• Plant in a damp cottage border with meadowsweet and moisture-loving grasses.
• Use small groups in a managed wet meadow to provide vertical accents among softer foliage.
• Combine with other locally native wetland flowers in a damp wildlife garden; do not use it for North American habitat planting.

Garden styles it suits: Cottage garden, Wildflower, Wildlife garden, Pollinator garden, Bog garden and New perennial (prairie).

Plant it with: Meadowsweet, Ragged robin, Marsh marigold, Water avens and Tufted hair grass.

Wildlife: Flowers provide nectar and pollen for bees and other insects. In North America, dense stands displace native wetland vegetation and alter wildlife habitat; its value to visiting insects does not outweigh that damage.

• Prolific self-seeding: remove spent spikes before seed develops and pull seedlings while small; deadheading does not make planting acceptable in invasive regions.
• Wilting or scorched leaves in dry soil: water deeply and, where cultivation is appropriate, move to reliably moist ground.
• Flopping stems in shade or overfed soil: provide more sun, stop high-nitrogen feeding and use discreet supports if needed.
• Leaf holes or stripped foliage from loosestrife leaf beetles: tolerate damage where possible; in North America these beetles may be deliberate biological controls and should not be sprayed.

Not known to be toxic to people, cats, dogs or horses. No particular part is established as a serious poisoning hazard, but it is not a food plant and should not be deliberately eaten or fed to animals.

Where it spreads. A serious invasive wetland plant across much of the United States and southern Canada, including the Great Lakes, northeastern states, Midwest and Pacific Northwest. Sale, planting or transport is restricted in many jurisdictions. It is also invasive in New Zealand. It is native to Britain and parts of Australia, so its status is not the same worldwide; check local rules and provenance.

• Rosebay willowherb / fireweed (Chamaenerion angustifolium)

  Has alternate leaves and larger, four-petalled pink flowers, followed by long seed pods releasing silky fluff; purple loosestrife usually has opposite or whorled leaves and flowers with six petals.

• Garden loosestrife (Lysimachia vulgaris)

  Has yellow, five-petalled flowers in branched clusters rather than dense purple-pink spikes; despite its common name, it belongs to a different plant family.

• European wand loosestrife (Lythrum virgatum)

  Usually has smoother, less hairy stems and leaves that narrow towards the base, rather than the broader, rounded or slightly clasping leaf bases of purple loosestrife. Hybrids can be difficult to distinguish.

Lythrum comes from the Greek word for blood, referring to the flowers' colour, while salicaria means willow-like, describing the narrow leaves.

Propagation. Where native and legal to grow, divide established clumps in spring. Seed germinates readily, but deliberate seed propagation is best avoided because escape is difficult to control. In invasive regions, do not propagate or share plants; follow local guidance for removal and disposal of roots and seed-bearing material.

Accepted botanical name

Lythrum salicaria

In the US and Canada

Introduced in Canada, the lower 48 states and St. Pierre and Miquelon.

Native range

Afghanistan, Albania, Algeria, Altay, Amur, Austria, Baltic States, Belarus, Belgium, Bulgaria, Central European Rus and China North-Central.

Measured

Soil pH 6.5 to 7. Average height 85 cm.

Sources: GBIF (CC BY 4.0), USDA PLANTS and Trefle (CC BY 4.0). The care and design notes are LandscapeGPT's own.