Canada thistle.

At a glance

Care level

Easy

Plant type

Perennial herb

Light

Full sun

Indoor light

Direct sun

Water

Every 7 to 14 days

Mature size

1 to 5 feet

Growth rate

Fast

Hardiness

USDA 3 to 9

Bloom time

Late spring to early autumn

Care

This plant thrives in full sun and tolerates a wide variety of soil conditions. Keep an eye out for rapid spread, as it can quickly overtake other plants in your garden. Regular removal of the entire root system is necessary to control its growth.

Watering

Water deeply when the top inch of soil feels dry.

Feeding

Generally does not require supplemental fertilizer.

Pruning

Cut back stems before they set seed to prevent spread.

Repotting

Repot only if you are keeping it in a container to manage its spread.

Soil

Well drained, fertile soil

Soil pH

6.0 to 7.5

Temperature

-30 to 35°C

Humidity

30 to 80%

Safety

People

The plant is not considered toxic, but the sharp spines can cause significant mechanical injury to skin.

Children

The sharp, spiny foliage poses a significant risk of puncture wounds to children handling the plant.

Contact allergens

The sharp spines and stiff hairs on the leaves and stems can cause skin irritation or dermatitis upon contact.

Airborne allergens

The windborne seeds have fine, feathery pappi that may cause respiratory irritation if inhaled in large quantities.

Edibility

While young leaves are sometimes consumed after careful preparation, the plant is generally considered unpalatable and potentially irritating.

How to identify it

• Spiny, lobed, lance shaped leaves
• Small, fragrant, purple flower heads
• Extensive creeping underground root system
• Stems are smooth and lack spines

Height

0.3 to 1.5m

Spread

0.3 to 0.6m

Leaves

green, deciduous

Flowers

purple, pink, white, 1 to 2cm

Fruit

brown, late summer

Problems to watch for

• Root SuckeringCommon
• RustCommon
• AphidsUncommon
• CaterpillarsUncommon

Growing it

Propagation

seed, division, root cuttings

When to plant

spring, autumn

Because this plant is so aggressive, it is best to avoid planting it near desirable garden species that it might crowd out.

Grows well with

• Common Yarrow · Both species thrive in similar open, sunny conditions.

Similar species

• Bull Thistle · It is a biennial species with much larger, more prominent spines.

Science

Family

Asteraceae

Genus

Cirsium

Order

Asterales

Class

Magnoliopsida

Native range

Europe and northern Asia.

Conservation

It is widely considered an invasive weed and is often subject to mandatory control measures in many regions.

It thrives in disturbed soils and open fields, using its deep root system to outcompete other vegetation.

Value to wildlife

nectar source, larval host plant

Pollinators

bees, butterflies, moths

Uses

pollinator habitat, wildlife forage

Taxonomy resolved against GBIF.

Questions people ask

How do I get rid of it?

Consistent, repeated removal of the entire root system is necessary to exhaust the plant's energy reserves.

Is it a good plant for a garden?

It is generally not recommended for home gardens due to its aggressive, invasive nature.

Where the name comes from

The genus name Cirsium comes from the Greek word for a thistle, while the species name arvense means of the field.

Good to know

• It is one of the most persistent and difficult weeds to eradicate from agricultural land.
• The root system can extend several meters deep into the soil.
• It produces thousands of seeds that are easily carried by the wind.
• Despite its reputation, it is a vital nectar source for many native pollinators.