Is this plant invasive?

It can spread readily by seed in favorable conditions.

Does it need full sun?

Yes, it prefers bright, sunny spots to bloom well.

Plant guide

Scarlet pimpernel.

Lysimachia arvensis

Pronounced ly-si-MA-ki-a ar-VEN-sis

Also known as Common Scarlet Pimpernel, Poor Man's Weather Glass, Scarlet Pimpernel, Shepherd's Calendar, Shepherd's Weather-Glass

The scarlet pimpernel is a charming, low growing plant often found in fields and gardens. It is well known for its small, star shaped flowers that close up when the sky turns cloudy. It is a hardy little plant that spreads easily in the right conditions.

Another photo of Scarlet pimpernel — Photo by Jose Gutierrez Higa (CC BY) via iNaturalist

At a glance

Care level: Easy
Plant type: Annual herb
Light: Full sun to partial shade
Indoor light: Bright indirect
Water: Every 3 to 7 days
Mature size: 12 to 20 inches
Growth rate: Fast
Hardiness: USDA 7 to 10
Bloom time: Late spring to early autumn

Care

Keep the soil consistently moist but not soggy to help this plant thrive. Watch for yellowing leaves, which often signal that the soil is staying too wet for too long. It prefers a spot with plenty of light to keep its stems from getting leggy.

Watering: Water when the top inch of soil feels dry to the touch.
Feeding: Apply a balanced liquid fertilizer once a month during the active growing season.
Pruning: Trim back leggy stems to encourage a bushier growth habit.
Repotting: Repot only if the plant becomes root bound or if you need to refresh the soil.
Soil: Well draining, fertile soil
Soil pH: 6.0 to 7.5
Temperature: -10 to 30°C
Humidity: 40 to 70%

Safety

People: The plant contains saponins which can cause irritation if consumed in large quantities.
Pets: This plant can be toxic to cats and dogs if ingested.
Children: Keep away from small children as the foliage and flowers are not meant for ingestion.
Contact allergens: The sap may cause mild skin irritation in sensitive individuals.
Airborne allergens: The pollen is not typically a significant allergen.
Edibility: This plant is not considered edible and may cause digestive upset if ingested.

How to identify it

Small star shaped orange or red flowers
Opposite, oval shaped green leaves
Square stems typical of primrose family
Flowers close during cloudy weather

Height: 0.1 to 0.3m
Spread: 0.2 to 0.5m
Leaves: green, deciduous
Flowers: orange, red, blue, 0.5 to 1.5cm

Problems to watch for

AphidsCommon
Powdery MildewCommon
DehydrationUncommon
Leaf SpotUncommon
Root RotUncommon
Slugs and SnailsUncommon

By symptom

Plant problems by symptom

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Growing it

Propagation: seed
When to plant: spring, autumn

Pair with other low growing annuals that appreciate well drained soil.

Grows well with

California Poppy · They share similar light and soil requirements.
Cornflower · They create a lovely meadow aesthetic together.

Similar species

Moneywort · It is a related species with a creeping habit.

Science

Family: Primulaceae
Genus: Lysimachia
Order: Ericales
Class: Magnoliopsida
Native range: Europe, Western Asia, and North Africa.
Conservation: It is considered a widespread weed in many regions and is not of conservation concern.

It thrives in open, disturbed habitats where it can quickly colonize bare soil.

Value to wildlife: nectar source
Pollinators: bees, butterflies
Uses: ground cover, rock garden plant

Through the year

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Flowering

Fruiting

Foliage

Ecological indicators

Salinity: 2/9
Soil reaction: 6/9
Light: 7/9
Temperature: 6/9
Moisture: 5/9
Nutrients: 5/9

Taxonomy resolved against GBIF.

Questions people ask

Is this plant invasive?: It can spread readily by seed in favorable conditions.
Does it need full sun?: Yes, it prefers bright, sunny spots to bloom well.

Where the name comes from

The genus name Lysimachia honors King Lysimachus of Thrace, while arvensis means of the field.

Good to know

It was formerly known as Anagallis arvensis.
The flowers are famously sensitive to light levels.
It is a common sight in disturbed soils and fields.

Also in

Plants toxic to cats and dogs

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This guide was generated with AI on September 26, 2026 and checked against GBIF taxonomy. It is general information, not horticultural or veterinary advice. Read about how we build these guides.