Isatis spp.

Europe, Mediterranean, and western and central Asia

Attracts Pollinators Attracts Butterflies Deer Resistant Drought Tolerant

HardinessZones 4–8

Overview

Isatis spp. is a genus of roughly thirty to eighty species of annual, biennial, and short-lived perennial herbs in the mustard family, Brassicaceae, native to Europe, the Mediterranean, and western and central Asia. The most familiar species, I. tinctoria (woad), was grown for centuries as the main European source of blue dye before imported indigo replaced it. Plants begin as a basal rosette of bluish-green, lance-shaped leaves, then send up a branched flowering stem 2 to 5 feet (0.6 to 1.5 m) tall carrying clouds of small, four-petalled yellow flowers. Flowering is followed by hanging, flattened, dark purple-brown winged seed pods, each holding a single seed. The genus is spread across temperate Eurasia, where the plants grow on dry banks, rocky slopes, and disturbed open ground. I. tinctoria has naturalised and become an aggressive weed in parts of the western United States, where it is listed as noxious in several states and forms dense stands on rangeland. The bluish foliage and yellow flowers last only briefly before the plant sets seed and dies, and self-sown seedlings can spread well beyond the original planting.

Native Range

Isatis spp. is native to temperate Europe, the Mediterranean region, and western and central Asia. The plants grow on dry, rocky, and disturbed open ground, and I. tinctoria has naturalised widely, including in North America where it is regarded as an invasive weed.

Suggested Uses

Isatis spp. is grown in dye gardens, herb collections, and historical or educational plantings for its long use as a blue dye source. The tall yellow flower sprays suit informal and wild borders where self-seeding is acceptable. Cut stems with young seed pods are used in dried arrangements.

How to Identify

First-year plants form a basal rosette of smooth, bluish-green, lance-shaped leaves with a pale midrib. In the second year a branched stem 2 to 5 feet (0.6 to 1.5 m) tall carries open sprays of small four-petalled yellow flowers, followed by hanging, flat, dark winged seed pods about 0.5 inch (1.3 cm) long, each with one seed. The blue-green clasping stem leaves, yellow flower sprays, and pendent dark pods identify Isatis spp. among the mustards.

Appearance

Size & Dimensions

Height2' - 5'

Width/Spread1' - 2'

Colors

Flower Colors

yellow

Foliage Colors

blue green

Fall Foliage Colors

no change

Bloom Information

Bloom Period

~4 weeks

Spring Summer

Flowering occurs in late spring and early summer, roughly May to June, when the branched stems carry many small yellow four-petalled flowers. The bloom period is short, lasting a few weeks before the pendulous seed pods form. Bees and other small insects visit the flowers.

Detailed Descriptions

Foliage Description

bluish-green

Growing Conditions

Sun Requirements

SunFull Sun

Requires 6-10 hours of direct sunlight daily

• Full Sun: 6+ hours of direct sunlight

• Partial Shade: 3-6 hours of direct sunlight

• Full Shade: Less than 3 hours of direct sunlight

Soil Requirements

pH Range6.0 - 8.0(Neutral)

357912

Soil Types

loam sand chalk rocky

Drainagewell drained

Water & Climate

Water Needs

Low

Frost Tolerance

hardy

Time to Maturity

2 years

Drought Tolerance

Drought tolerant when established

Care & Maintenance

Care Guide

Isatis spp. grows in full sun in well-drained, even poor or rocky soils, and tolerates dry, limy, and disturbed ground. The genus is hardy in roughly USDA zones 4 to 8, with biennial species completing growth over two seasons and dying after setting seed. Plants are drought-tolerant once established and need little care. Because I. tinctoria self-seeds heavily and is invasive in some regions, removing seed pods before they ripen limits unwanted spread. Propagation is by seed sown in spring or early summer, which germinates readily.

Pruning

Cutting flowering stems before the seed pods ripen reduces self-seeding, which matters most for the weedy I. tinctoria. Biennial plants die after flowering and are removed once seed has been collected or cut. No other pruning is needed.