Herb · Rutaceae
Rue
Ruta graveolens
Cool blue-green foliage on a low woody mound, lit each summer by small mustard-yellow flowers.
Full sun
Every 7–14 days
80–120 days
USDA 4–9
Overview
Rue reads as a soft cushion of cool blue-green from a distance, an evergreen herb whose color is unusual among garden foliage. Up close it is a woody-based perennial, building a rounded mound 2 to 3 feet (60 to 90 cm) tall, its lower stems hardening and turning bark-like with age. Through summer it lifts small, mustard-yellow flowers above the foliage. It has a long past in old gardens and apothecaries, and today grows as an ornamental and a host plant for swallowtail butterflies.
The blue-green cast is the plant's signature, holding through the seasons on foliage that stays put year-round. It carries that color on lean, sharply drained soil and full sun, the conditions that keep the mound dense and firm, and it holds its form through drought and cold to 5°F (-15°C). Rich, wet ground loosens the whole plant, and soggy roots are what bring it down.
For all that cool, quiet color, the plant carries a sting you cannot see. Its tissue is rich in furanocoumarins, and the clear sap that seeps from a cut or crushed stem turns phototoxic in sunlight, raising painful, blistering burns on skin. That hidden trait is why the plant is handled in gloves and long sleeves, set away from pets and children, and kept off the plate.
Care
The short version: rue keeps its dense blue-green mound in full sun and lean, sharply drained soil, watered deeply but only once the top half has dried. It is cut back hard each spring, fed little or not at all, and handled in gloves because the sap blisters skin.
Light
Rue holds its dense, tight blue-green mound only in full sun, where bright light keeps the foliage firm and close-set. It will take some afternoon shade, but a shaded plant loses that compactness, stretching pale and open as the stems reach and flop.
Water
The blue-green foliage stays firm on a dry regime, watered deeply but only every 7 to 14 days, with the top half of the soil left to dry between drinks. Once established the plant draws through drought without wilting, and it turns against wet feet, where soggy ground rots the roots out from under the mound.
Soil
Rue sets its woody base into lean, sharply drained soil, sandy or loamy, ground that most herbs only tolerate. Any pH from 6.0 to 7.8 suits it, and poor soil actually tightens the plant, keeping the blue-green growth compact and the stems sturdy rather than soft.
Temperature
The evergreen foliage rides out extremes, taking heat to 95°F (35°C) and cold to 5°F (-15°C) without dropping its color. That toughness lets the same woody-based mound return year after year in zones 4 through 9, its blue-green holding through winter.
Humidity
Rue keeps its best color and firmest form in dry air, around 30% to 50%, matching its Mediterranean origins. Damp, stagnant air settles on the foliage and invites the rot the plant is prone to, dulling and softening the tight blue-green surface.
Feeding
Fed lightly in spring or not at all, rue keeps its dense, close mound. It thrives in poor soil, and too much fertilizer pushes weak, floppy stems that outrun the plant's frame, splaying open and breaking the rounded blue-green outline.
Pruning
Cut hard in early spring, rue answers by pushing fresh, compact blue-green shoots from its old woody base, and a trim after flowering holds the rounded shape. Gloves, long sleeves, and a cloudy day are needed, because the sap on skin turns to blistering burns under the sun.
Repotting
In a container, the woody-based plant needs moving up only every 2 to 3 years. The signs show plainly: roots circling the pot or pushing through the drainage holes, water running straight past the rootball, or new growth coming in sparse and stunted.
Size & spacing
Rue builds a rounded, mounded shrub 2 to 3 feet (60 to 90 cm) tall and 18 to 24 inches (45 to 60 cm) across, its blue-green foliage densest toward the center. Set each plant that far apart so the mounds stay distinct and air moves through the foliage.
When to plant
Rue is a hardy perennial that comes on slowly. Seed is sown indoors in late winter, young plants go out in spring once frost has passed, and the small mustard-yellow flowers open above the blue-green foliage through summer, roughly June into August.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: 6-8 weeks before last spring frost
- Plant: spring, after danger of frost has passed
- Harvest: Mustard-yellow flowers in summer (roughly June-August); grown ornamentally - do NOT harvest the foliage for eating due to toxicity and phototoxic sap.
Hardiness
USDA hardiness zones: 4–9
Temperature: down to -34°C
Troubleshooting
Rue shows its troubles plainly, in the color and set of its foliage, and most of them come from too much water or a skipped pruning. The signs below describe what a struggling plant actually looks like. Match what you see on the leaves, stems, or roots, then act before the damage spreads.
Wilting foliage, yellowing color, and a collapsing mound
When the blue-green mound wilts, yellows, and sinks in on itself, the trouble is root rot at the base, almost always from wet feet. Below ground the roots turn brown, soft, and mushy as soggy, heavy soil suffocates them. The water is cut back at once, grit or sand is worked in to sharpen drainage, and any plant with brown, mushy roots is lifted out and discarded.
Fix:
- Reduce watering immediately and allow soil to dry
- Improve drainage by amending with grit or sand, or relocate to a raised/well-drained site
- Remove and discard severely rotted plants
Bare woody stems with blue-green leaves only at the tips
A plant gone leggy and unpruned shows a bare, hardened, bark-like base with its blue-green leaves clustered only at the stem ends. The lower foliage has dropped away, and weak light draws the stems out longer and thinner. A hard cut in early spring forces fresh, compact shoots from the base, and full sun keeps the new growth tight rather than stretched.
Fix:
- Cut back hard in early spring to encourage compact new shoots
- Ensure the plant receives full sun
Blue-green leaves stripped by banded caterpillars
Blue-green foliage chewed down to ragged stubs is the work of swallowtail butterfly larvae, the green-and-black banded caterpillars feeding openly on rue, one of their favorite host plants. Most gardeners leave the caterpillars in place and accept the chewed leaves as part of raising butterflies. Where the damage is too much, the caterpillars are picked off and moved by hand, and broad sprays are avoided.
Fix:
- Tolerate the caterpillars where butterflies are desired, as rue is a valued host plant
- Hand-pick and relocate caterpillars if damage is unacceptable
Toxicity
The cool blue-green surface of rue hides a genuinely hazardous sap. Its tissue holds furanocoumarins that turn phototoxic in sunlight, so sap on skin followed by sun raises painful, blistering burns, and the foliage is toxic to cats, dogs, horses, and grazing animals if eaten. No part of it is safe to eat, so it is handled in gloves and kept well away from pets and children.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Moderate
A cat that bites the blue-green foliage or picks up the sap drools, vomits, and passes loose stools, then turns dull and withdrawn, while the skin it touched reddens and grows sore.
Dogs Moderate
A dog that chews the leaves drools and brings on vomiting and diarrhea, often with flagging energy, and the sap leaves a raw, inflamed patch of skin wherever it settled.
Horses Moderate
Rue sours a horse's stomach when grazed, and the sap raises blistered, inflamed skin, showing worst on the pale, thin-haired, sun-exposed skin of the muzzle and lower legs.
Rabbits Toxic
The blue-green leaves are unsafe for rabbits to eat, their furanocoumarins turning a curious nibble into a sore, upset gut and reddened, irritated skin.
Livestock Toxic
Grazing animals that eat rue take on digestive upset and photosensitization, their sun-exposed skin reddening and blistering once the compounds build up in the body.
Birds Caution
The bitter blue-green foliage gives birds little reason to peck at it, and the same phototoxic compounds mark it as a plant to keep out of reach rather than offer.
People Caution
No part of the plant is safe to eat, and the sharpest hazard is to the skin: clear sap on the hands, then sunlight, and painful blisters and burns rise where it touched. Gloves and long sleeves go on for every handling and pruning.
Always handle rue with gloves and long sleeves, and avoid contact on sunny days, because sap on skin followed by sunlight causes painful blistering burns (worse than parsley or celery). The plant must NOT be eaten by people or animals and is not used as a modern culinary herb.
Helpline numbers
United States
ASPCA Animal Poison Control: (888) 426-4435 (fee applies)
Poison Control: (800) 222-1222
Canada
Poison Centres: 1-844-764-7669
Varieties
Two named forms stand out by the color and marking of their leaves alone. Here is what each looks like up close.
'Jackman's Blue'
A compact form whose foliage takes on an unusually deep blue-green, a richer, steelier cast that it holds all season.
'Variegata'
Its blue-green leaves are splashed across the blade and edged along the margins with creamy white, breaking the color into a soft, mottled pattern.
How to propagate
More of that blue-green foliage comes readily from seed or cuttings, though both ask for patience, and cuttings ask for gloves against the sap.
SeedModerate
Start indoors 6-8 weeks before last frost, or sow outdoors in spring; germination can be slow · Several weeks to germinate and a full season to establish
Rue is grown from seed indoors 6 to 8 weeks before the last frost, or sown outdoors in spring. The seed is pressed onto the soil surface rather than buried, since light triggers germination, and the surface is kept evenly moist until the first green seedlings show. Germination runs slow, and the plant takes a full season to fill out into its blue-green mound.
Stem cuttingsModerate
Take softwood or semi-ripe cuttings in late spring to summer · 4-8 weeks to root
Softwood or semi-ripe cuttings are taken from late spring through summer, their blue-green stems set into a gritty, well-drained mix where they root in 4 to 8 weeks. Gloves stay on through the cutting and handling, since the sap that beads from a cut stem burns skin in sunlight.
Myths
Rue is a safe culinary herb you can cook with as freely as other Mediterranean herbs.
Old recipes did use it, but its bitter blue-green foliage has no place in the modern kitchen. Rue is toxic to eat, and its sap sets off severe sun-triggered skin reactions, so it holds its place today as an ornamental and historical herb, not a culinary one.