White snakeroot
Ageratina altissima · Asteraceae (Compositae) - Aster or Sunflower Family
Fluffy heads of pure white, like a white ageratum, light up dry woodland in early autumn, when little else flowers in shade.
- Size
- 3½ ft tall, 2½ ft wide
- Zones
- 3 to 8
- Light
- Part sun to shade
- Flowers
- Late August to mid October
- Pets
- Toxic to people and horses
- Job in a bed
- Drift
A bed built around white snakeroot
The planner designed this bed around white snakeroot when this page was made: a 12 × 6 ft (3.7 × 1.8 m) cottage garden in zone 6, part shade. It chose the other plants from the 513 that suit those conditions, and says below why white snakeroot stands where it does.
The plants
- 1 White snakeroot × 3, white
- 2 Moor grass 'Greenlee' × 3, tan
- 3 Dalmatian bellflower × 9, purple
- 4 Foxglove × 3, pink
- 5 Inkberry 'Gem Box' × 1, dark green leaves
- 6 Blue wood aster × 5, lavender
- 7 Yellow trout lily × 10, yellow
- 8 Glory-of-the-snow × 12, blue
Why this plant
- The only dense clusters. The others are spires, bells and daisies. Shape reads at a distance, where colours blur.
- A classic partner for blue wood aster. Both are on this plan. A pairing gardeners have proved outranks a general rule.
- The only white. White sits between stronger colours and calms them.
Why here
- Anchors a back corner. At 3½ ft it is among the tallest plants here, and the tallest close the ends of the bed, where they stand in front of nothing.
- A step in the back row. Next to inkberry 'Gem Box' (2½ ft): no two neighbours there are the same height, so the back steps up and down like a skyline.
- 3 plants in one group. Massed, one kind reads as a single shape. Scattered, it would look spotty.
Why now
- In flower for 7 weeks. From late August to mid October.
- Fills a gap. For two of those weeks nothing else on the plan is in flower.
Planned as a cottage garden: 24 of the 24 plants (100%), bulbs aside, stand alone or in groups of no more than five, small enough for the kinds to mingle. Cool colours (purple, pink, lavender and blue) with yellow as the warm note, and white and tan to calm things between them. Foxglove and white snakeroot anchor the back corners of the bed: the tallest plants on the plan stand there.
The planner's own design check: 10 Excellent, 6 Good, 1 Satisfactory. It flags one thing to watch: White snakeroot is highly poisonous to people and horses. Foxglove is highly poisonous to people, dogs, cats and horses.
Open this bed in the planner Change its size, the sun or the style, move plants about, or swap one for another.
Where white snakeroot goes in a border
At 3½ ft it belongs in the middle of a bed. In the planner's ten height classes it is moderately tall (3 to 4 ft).
Its job: drift. Planted in a small group of three or five, and repeated further along in the other number.
A native that flowers in dry shade in September is valuable, but it seeds everywhere. Cut it down as the flowers fade. It poisoned the milk of pioneer cattle.
- Autumn flowers in dry shade
- Native woodland edges
- Late nectar for pollinators
What to plant with white snakeroot
Plants recorded as good partners for it, and what each pairing does:
- Blue wood aster: in flower together for five weeks, from mid September.
- Zigzag goldenrod: in flower together for five weeks, from late August.
- Christmas fern: its dark green leaves are a setting for the flowers, before and after; low enough (20 in) to stand in front of it.
Questions about white snakeroot
Does white snakeroot need sun or shade?
White snakeroot grows in part sun (4 to 6 hours of sun, including some afternoon sun), part shade (2 to 4 hours of sun, mostly in the morning) and shade (less than 2 hours of direct sun, or bright dappled light).
It wants ordinary well-drained soil, of average fertility, and a good soak in dry spells.
What zones does white snakeroot grow in?
USDA zones 3 to 8. It is hardy to zone 3, which means it survives winter nights down to about −30 °F (−34 °C).
Above zone 8 it struggles: the zones measure winter cold, but a plant's top zone usually means the summers are too hot or the winters too mild for it to rest.
Gardening in Canada? Use the USDA zone for your town: Canada's own zone map uses different numbers.
When does white snakeroot flower in my zone?
In zone 5 it flowers late August to mid October. As a summer flower it moves about half a week for every zone: earlier where it is warmer.
| Zone 3 | Early September to mid October |
|---|---|
| Zone 4 | Early September to mid October |
| Zone 5 | Late August to mid October |
| Zone 6 | Late August to mid October |
| Zone 7 | Late August to mid October |
| Zone 8 | Late August to mid October |
Worked out from Hopkins' bioclimatic law: spring arrives about four days later for each degree of latitude north, and one zone is roughly two degrees. A warm spring or a cold one moves any garden two weeks either way.
How far apart should white snakeroot be planted?
24 in (60 cm) apart, from the middle of one plant to the middle of the next. That is a little closer than its full width of 2½ ft (75 cm), so a group knits into one patch.
It grows 3½ ft (100 cm) tall. Give it the full distance from anything taller, or it will lean out towards the light.
How many white snakeroot plants do I need?
At 24 in apart, set out in staggered rows:
- 3 to fill 10 square feet (about 1 m²)
- 6 for a single row 10 ft (3 m) long
- 10 to fill a 4 × 8 ft bed on its own
In a mixed border, plant it as a group of three or five, with a second group further along in the other number.
In the bed above, the planner used three in one group.
Is white snakeroot poisonous?
Yes: severe toxicity to people and horses. All parts are highly poisonous if eaten, especially to horses and other grazing animals.
Rated from NC State Extension and the ASPCA. Where references disagree, the more cautious rating is used.
Do deer eat white snakeroot?
Mostly not. NC State lists it as resistant to deer. No plant is proof against a hungry deer in a hard winter, but this one is low on the menu.
Will white snakeroot spread?
It seeds itself about; the risk of it taking over is high. Self-seeds heavily on wind-blown seed. Cut it down as soon as the flowers fade.
What flowers after white snakeroot?
White snakeroot is over by mid October. These start as it finishes and carry the same spot on, so it never goes flat:
- Japanese onion 'Ozawa': from early October to early November, 10 in tall, to stand in front of it and cover its fading stems.
- Bodnant viburnum 'Dawn': from early November to late December, 10 ft tall.
- Common witch hazel: from mid October to late November, 15 ft tall.
What does white snakeroot look like in winter?
White snakeroot dies down, but leave the stems standing: the seed heads stand through the winter, and songbirds eat the seed. Cut them down in early spring, before the new shoots come.
Is white snakeroot good for bees and butterflies?
White snakeroot is visited by pollinators, butterflies and songbirds.
It is native to the United States.
More Ageratina
- White snakeroot 'Chocolate' Ageratina altissima 'Chocolate'
In these guides
- Deer-resistant plants for shade
- Native perennials for pollinators
- Native plants for shade
- Plants for butterflies
- Plants that feed garden birds
- Plants that flower in fall
- Plants for a white garden
Zone guides: Zone 3 · Zone 4 · Zone 5 · Zone 6 · Zone 7 · Zone 8
Every fact on record
- Type
- Perennial
- Size
- 3½ ft (100 cm) tall, 2½ ft (75 cm) wide
- Plant
- 24 in (60 cm) apart
- Height class
- Moderately tall (3 to 4 ft)
- Place in the bed
- Middle
- Flowers
- Late August to mid October; dense clusters; white
- Leaves
- Medium texture, herbaceous; green
- Shape
- Upright, self-seeding
- Hardy in
- USDA zones 3 to 8
- Light
- Part sun, part shade, shade
- Soil
- Average, well-drained
- Water
- Moderate
- Feeding
- None needed
- Family
- Asteraceae (Compositae) - Aster or Sunflower Family
- Comes from
- Native to woodlands in Eastern United States
- Attracts
- Pollinators, butterflies, songbirds
- Stands up to
- Deer
Checked against NC State Extension, ASPCA poison lists and VASCAN (plants of Canada). Flowering times are for zone 5: later in colder zones, earlier in warmer ones. Toxicity and spread information is a general gardening guide, not veterinary or medical advice, and invasive status varies by region. If a person or pet eats a plant, contact poison control or a vet. Check your local invasive species list before planting anything with a spread warning.