Slender mountain mint
Pycnanthemum tenuifolium · Lamiaceae
Needle-fine leaves and flat clusters of small white flowers make a soft, airy, rounded plant in midsummer. It tolerates drier soil than other mountain mints.
- Size
- 2½ ft tall, 24 in wide
- Zones
- 4 to 8
- Light
- Full sun to part sun
- Flowers
- Early July to late August
- Pets
- No toxicity recorded
- Job in a bed
- Drift
A bed built around slender mountain mint
The planner designed this bed around slender mountain mint when this page was made: a 12 × 6 ft (3.7 × 1.8 m) naturalistic planting in zone 6, full sun. It chose the other plants from the 966 that suit those conditions, and says below why slender mountain mint stands where it does.
The plants
- 1 Slender mountain mint × 3, white
- 2 Winter heath × 6, pink
- 3 Coral bells 'Firefly' × 5, red
- 4 Smooth aster 'Bluebird' × 3, blue
- 5 Silver sage × 1, silver leaves
- 6 Golden ragwort × 14, yellow
- 7 Allium 'Ambassador' × 3, purple
- 8 Tulip 'Ballerina' × 7, orange
Why this plant
- The only dense clusters. The others are bells, airy sprays, daisies and others. Shape reads at a distance, where colours blur.
- The only white. White sits between stronger colours and calms them.
Why here
- At the back. At 2½ ft it is seen over golden ragwort (20 in) in front of it, and has nothing behind it to hide.
- A step in the back row. Next to smooth aster 'Bluebird' (3½ ft) and allium 'Ambassador' (4 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 9 weeks. From early July to late August.
- Fills a gap. For five of those weeks nothing else on the plan is in flower.
Planned as a naturalistic planting: low grasses and groundcovers, the matrix, make up 50% of the planted ground. The bed is balanced: the left half carries 47% of the visual weight and the right half 53%. The bold leaves of silver sage are set against the fine leaves of winter heath, the kind of contrast that keeps a bed from blurring.
The planner's own design check: 11 Excellent, 6 Good. It flags one thing to watch: Nothing on this plan is highly poisonous or an aggressive spreader.
Open this bed in the planner Change its size, the sun or the style, move plants about, or swap one for another.
Where slender mountain mint goes in a border
At 2½ ft it belongs in the middle of a bed. In the planner's ten height classes it is medium (24 in to 2½ ft).
Its job: drift. Planted in a small group of three or five, and repeated further along in the other number.
The texture of a small conifer with the flowers of a herb. It makes a quiet, fine-leaved background for bold coneflowers and milkweeds.
- Fine texture in a prairie planting
- Pollinator gardens
- Dry to average soil
What to plant with slender mountain mint
Plants recorded as good partners for it, and what each pairing does:
- Butterfly weed: in flower together for six weeks, from early July.
- Little bluestem: its blue-green leaves are a setting for the flowers, before and after.
- Purple coneflower: in flower together for nine weeks, from early July.
Questions about slender mountain mint
Does slender mountain mint need sun or shade?
Slender mountain mint grows in full sun (6 or more hours of direct sun a day) and part sun (4 to 6 hours of sun, including some afternoon sun).
It wants ordinary well-drained soil, of average fertility, and little watering once settled. Once settled in, it stands drought.
What zones does slender mountain mint grow in?
USDA zones 4 to 8. It is hardy to zone 4, which means it survives winter nights down to about −20 °F (−29 °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 slender mountain mint flower in my zone?
In zone 5 it flowers early July to late August. As a summer flower it moves about half a week for every zone: earlier where it is warmer.
| Zone 4 | Mid July to early September |
|---|---|
| Zone 5 | Early July to late August |
| Zone 6 | Early July to late August |
| Zone 7 | Late June to late August |
| Zone 8 | Late June to late August |
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 slender mountain mint be planted?
18 in (45 cm) apart, from the middle of one plant to the middle of the next. That is a little closer than its full width of 24 in (60 cm), so a group knits into one patch.
It grows 2½ ft (75 cm) tall. Give it the full distance from anything taller, or it will lean out towards the light.
How many slender mountain mint plants do I need?
At 18 in apart, set out in staggered rows:
- 5 to fill 10 square feet (about 1 m²)
- 7 for a single row 10 ft (3 m) long
- 17 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 slender mountain mint poisonous to dogs and cats?
No toxicity is recorded for dogs, cats, horses or people, according to NC State Extension.
"Not toxic" is not "edible": a pet that eats a lot of any plant can be sick.
Do deer eat slender mountain mint?
They may. NC State does not list it as resistant to deer or rabbits. Where they visit, protect it, or plant it among ones they avoid.
Will slender mountain mint spread?
It spreads sideways, by runners or roots; the risk of it taking over is moderate. Spreads by underground runners and can crowd its neighbours. Dig out the edges or divide every 2 to 3 years.
What flowers after slender mountain mint?
Slender mountain mint is over by late August. These start as it finishes and carry the same spot on, so it never goes flat:
- Garden chrysanthemum: from mid September to late October, 24 in tall.
- White wood aster: from late August to early October, 24 in tall.
- Bottle gentian: from late August to early October, 20 in tall, to stand in front of it and cover its fading stems.
What does slender mountain mint look like in winter?
Slender mountain mint dies down to the ground and there is nothing to see until spring. The old stems can be cut in autumn, or left until spring as shelter for overwintering insects.
Is slender mountain mint good for bees and butterflies?
Slender mountain mint is visited by pollinators, bees and butterflies.
It is a host plant: the caterpillars of wavy-lined emerald feed on its leaves. A butterfly can only lay its eggs on its host plants, so these do more for it than any nectar flower.
It is native to the United States and Canada.
More Pycnanthemum
- Mountain mint Pycnanthemum muticum
In these guides
- Plants for slopes and banks
- Drought-tolerant perennials for full sun
- Pet-safe plants for a sunny border
- Native perennials for pollinators
- Garden plants native to Canada
- Plants for butterflies
- Plants for bees
- Plants for a white garden
- Host plants for the wavy-lined emerald
Every fact on record
- Type
- Perennial
- Size
- 2½ ft (75 cm) tall, 24 in (60 cm) wide
- Plant
- 18 in (45 cm) apart
- Height class
- Medium (24 in to 2½ ft)
- Place in the bed
- Middle
- Flowers
- Early July to late August; dense clusters; white
- Leaves
- Fine texture, herbaceous; green
- Shape
- Upright, spreading
- Hardy in
- USDA zones 4 to 8
- Light
- Full sun, part sun
- Soil
- Average, well-drained
- Water
- Low
- Feeding
- None needed
- Family
- Lamiaceae
- Comes from
- Eastern Canada to Central & Eastern U.S.A
- Attracts
- Pollinators, bees, butterflies
- Host plant for
- Wavy-lined emerald
- Stands up to
- Drought, erosion
Checked against NC State Extension. 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.