Common rue (Ruta graveolens, L. 1753)

Identity

Common rue (Ruta graveolens L., 1753), also called herb-of-grace, is an aromatic subshrub in the genus Ruta and the citrus family, Rutaceae. Plants of the World Online and the International Plant Names Index confirm the accepted name, published in Linnaeus’s Species Plantarum. ^{1 2}

Its sap offers a striking distinction from nearby border herbs: touching it and then exposing the skin to sunlight can cause burns, a reaction not produced by neighboring thyme or lavender. The strong, acrid smell of crushed foliage is unpleasant to some people, yet valued in traditional cooking in the Balkans and northern Italy. The English name herb-of-grace recalls an old liturgical use: sprigs were used to sprinkle holy water in some Catholic rites. ^{4 6}

French botanical records in the INPN list this species as rue odorante, with rue officinale and rue des jardins among its other French vernacular names. ⁷

Recognition

A low, rounded clump reaches 50 cm to 1 m (20–39 in) tall with a similar spread, supported by stems that become woody at the base. Its evergreen, blue-gray to glaucous leaves divide two or three times into small, rounded lobes. This gives it a different appearance from green-leaved Mediterranean herbs such as thyme and oregano. ^{3 4}

Summer brings cymes of yellow flowers 1.5–2 cm (0.6–0.8 in) across, with four or five petals whose edges are usually entire or only slightly toothed. Petal margins identify Ruta graveolens more reliably than foliage color alone: Ruta chalepensis has conspicuous fringes along the full length of its petals. ³

After flowering, each flower develops into a brown capsule with four or five rounded lobes. In Ruta chalepensis, the lobes are pointed and project conspicuously, a useful distinction even after the flowers have faded. ⁴

Useful taxonomy

Linnaeus published Ruta graveolens L. in 1753 in Species Plantarum, and it remains the accepted name. Several botanical synonyms and historical varieties are now included under it:

• Ruta hortensis Mill., a historical synonym
• Ruta officinalis Pall., a historical synonym
• Ruta divaricata Ten., a historical synonym
• Ruta diversifolia Wender., a historical synonym
• Ruta macrophylla Sol., a historical synonym
• Ruta graveolens f. variegata (Weston) Zabel, a historical variegated form

For everyday identification, the practical issue is the frequent confusion with Ruta chalepensis, fringed rue. It is a close but distinct species, used more often in Ethiopian cooking than common rue. ^{1 2}

Habitat and range

From the Balkan Peninsula to Crimea, the native distribution centers on southeastern Europe. Continuous ornamental, culinary and medicinal cultivation since antiquity subsequently helped the plant naturalize widely around the Mediterranean. ^{1 7}

Ruta graveolens grows on rocky limestone slopes, old walls, dry banks and sunny lowland waste ground, in poor, well-drained soil with a pH of 6–8. Winter waterlogging is far more damaging than dry cold: roots can rot within weeks in soil that stays saturated. ^{4 3}

Beyond its native range, common rue is widely grown in dry herb gardens across temperate regions, including France and the United Kingdom. It is rated hardy to USDA zone 4, depending on winter drainage. ^{4 3}

Life cycle and reproduction

June and July flowering attracts pollinators. Common rue also hosts caterpillars of several swallowtails, including the Old World, anise and giant swallowtails. After pollination, each flower produces a brown seed capsule. ⁴

Propagation uses spring sowing, semi-ripe cuttings taken in late summer, or division of an established clump. Cuttings root in 3–5 weeks in a free-draining medium, taking considerably longer in heavy, wet soil. ⁶

A clump remains fully productive for about 3–5 years. After that, its base becomes woody and its center opens up, reducing foliage density and flowering. Cutting back or dividing the clump every three to four years helps prevent this decline in vigor. ⁶

Uses and risks

Year-round blue-gray foliage gives common rue its main role as an ornamental for low borders, rock gardens and dry beds. It also has a long culinary history around the Mediterranean, notably in grappa alla ruta from Istria and northern Italy, although this use is now marginal. ^{4 6}

The main hazard affects the skin. Sap contains photosensitizing furanocoumarins, including psoralen and chalepensin; sunlight exposure after contact can cause phytophotodermatitis, with blisters resembling true burns. Cases include young children who merely touched the plant in a garden. ^{8 4}

Leaves also contain quinolone alkaloids documented for natural fungicidal properties. Large ingestions can cause digestive pain and vomiting. Common rue has a history as a traditional abortifacient and must be avoided during pregnancy. ^{9 6}

Varieties, look-alikes and related plants

Horticultural selections sold as Ruta graveolens are forms of the same species. ‘Jackman’s Blue’, listed by the Royal Horticultural Society, has deeper blue-gray foliage and a compact habit. ‘Variegata’ has cream-variegated leaves and is less vigorous than the wild type. ‘Blue Beauty’ is grown mainly for its particularly intense glaucous foliage. The name prostrata, used at subspecies rank, denotes a dwarf, trailing form for borders and rock gardens. ³

Fringed rue, Ruta chalepensis, is the most frequent look-alike. Both have bluish foliage and contact toxicity, but fully fringed petals and pointed fruit lobes identify Ruta chalepensis. Common rue instead has entire or slightly toothed petals and rounded fruit lobes. ^{3 4}

Other silver-leaved Mediterranean herbs, including some lavenders and wormwood, can resemble common rue from a distance. Its strong, distinctive odor when crushed and characteristic blue-gray foliage provide the quickest field clues. ⁴

Main sources

• Plants of the World Online / Kew, Ruta graveolens L.
• International Plant Names Index, Ruta graveolens L.
• Royal Horticultural Society, Ruta graveolens - common rue
• NC State Extension Gardener Plant Toolbox, Ruta graveolens
• Missouri Botanical Garden Plant Finder, Ruta graveolens
• Plants For A Future, Ruta graveolens
• INPN, Ruta graveolens L. 1753, common rue
• Furniss & Adams, 2007, herb of grace phytophotodermatitis mimicking burn injury
• Oliva et al., 2003, natural fungicides and quinolone alkaloid from Ruta graveolens leaves