Rocky Mountain penstemon
Penstemon strictus ·
One-sided spikes of deep royal blue to purple flowers in early summer, above a mat of evergreen leaves. One of the easiest western penstemons.
- Size
- 2½ ft tall, 18 in wide
- Zones
- 4 to 9
- Light
- Full sun
- Flowers
- Late May to early July
- Pets
- Not checked
- Job in a bed
- Accent
A bed built around Rocky Mountain penstemon
The planner designed this bed around Rocky Mountain penstemon when this page was made: a 12 × 6 ft (3.7 × 1.8 m) cottage garden in zone 6, full sun. It chose the other plants from the 715 that suit those conditions and its dry soil, and says below why Rocky Mountain penstemon stands where it does.
The plants
- 1 Rocky Mountain penstemon × 3, blue
- 2 Winter heath × 5, pink
- 3 October daphne × 3, pink
- 4 False sunflower 'Summer Nights' × 3, yellow
- 5 Ironweed 'Iron Butterfly' × 4, purple
- 6 Silver sage × 1, silver leaves
- 7 Schubert's allium × 4, pink
- 8 Lady tulip × 12, white
Why this plant
- The only spires. The others are bells, dense clusters, daisies and others. Shape reads at a distance, where colours blur.
- The only blue.
Why here
- In the middle. At 2½ ft it is seen over silver sage (12 in) in front of it.
- 3 plants in 3 places. An accent is a punctuation mark: the repeat along the bed gives it rhythm.
Why now
- In flower for six weeks. From late May to early July.
- Part of the bed's peak. With Schubert's allium in late May: 2 kinds at once. No week has more.
Planned as a cottage garden: 19 of the 19 plants (100%), bulbs aside, stand alone or in groups of no more than five, small enough for the kinds to mingle. Pink is the colour seen most, on 3 of the 7 kinds grown for their flowers. The bed is balanced: the left half carries 45% of the visual weight and the right half 55%.
The planner's own design check: 12 Excellent, 5 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 Rocky Mountain penstemon 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: accent. An upright plant used singly, like a punctuation mark, and repeated at intervals to give the bed rhythm.
An intense blue that few dry-garden plants can match. It lives longest in lean, unwatered soil; rich soil and summer irrigation shorten its life.
- Dry, sunny gardens
- Blue spires for low-water planting
- Hummingbird and native bee gardens
What to plant with Rocky Mountain penstemon
Plants recorded as good partners for it, and what each pairing does:
- Blanket flower: blue against yellow, opposite each other on the colour wheel, in flower together for four weeks; a low mound at the foot of its spires; happy in the same dry soil.
- Blue flax: in flower together for six weeks, from late May; happy in the same dry soil.
- Yarrow 'Moonshine': blue against yellow, opposite each other on the colour wheel, in flower together for four weeks; happy in the same dry soil.
Questions about Rocky Mountain penstemon
Does Rocky Mountain penstemon need sun or shade?
Rocky Mountain penstemon grows in full sun (6 or more hours of direct sun a day).
It wants dry, sharply drained soil, on the lean side, with no feeding, and little watering once settled.
What zones does Rocky Mountain penstemon grow in?
USDA zones 4 to 9. It is hardy to zone 4, which means it survives winter nights down to about −20 °F (−29 °C).
Gardening in Canada? Use the USDA zone for your town: Canada's own zone map uses different numbers.
When does Rocky Mountain penstemon flower in my zone?
In zone 5 it flowers late May to early July. As a spring flower it moves about a week for every zone: earlier where it is warmer, later where it is colder.
| Zone 4 | Early June to mid July |
|---|---|
| Zone 5 | Late May to early July |
| Zone 6 | Late May to early July |
| Zone 7 | Mid May to late June |
| Zone 8 | Early May to late June |
| Zone 9 | Early May to late June |
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 Rocky Mountain penstemon be planted?
16 in (40 cm) apart, from the middle of one plant to the middle of the next. That is a little closer than its full width of 18 in (45 cm), so a group knits into one patch.
It grows 2½ ft (70 cm) tall. Give it the full distance from anything taller, or it will lean out towards the light.
How many Rocky Mountain penstemon plants do I need?
At 16 in apart, set out in staggered rows:
- 7 to fill 10 square feet (about 1 m²)
- 8 for a single row 10 ft (3 m) long
- 21 to fill a 4 × 8 ft bed on its own
In a mixed border, plant it as one at a time, repeated along the bed at intervals; a slender one reads better as a clump of three.
In the bed above, the planner used three in three groups.
Is there a native plant like Rocky Mountain penstemon?
Rocky Mountain penstemon is not native to North America. These North American natives come closest to doing its job:
- Firecracker penstemon: the same job in a bed, the same size and the same flowering time; red, not blue, keeps its leaves in winter.
- Blue flag iris: the same job in a bed, the same size, the same flowering time and the same colour; wants wet soil.
Matched the way the planner's Swap button matches plants: the same job in a bed, then size, then flowering time.
What flowers after Rocky Mountain penstemon?
Rocky Mountain penstemon is over by early July. These start as it finishes and carry the same spot on, so it never goes flat:
- Black-eyed Susan: from mid July to mid September, 24 in tall.
- Butterfly weed: from late June to early August, 24 in tall.
- Daylily: from late June to early August, 2½ ft tall.
What does Rocky Mountain penstemon look like in winter?
Rocky Mountain penstemon holds some of its leaves into winter, more in a mild year or a sheltered spot, and they look tired by March. Cut the old ones off as new growth starts.
More Penstemon
- Beardtongue 'Dark Towers' Penstemon 'Dark Towers'
- Beardtongue 'Husker Red' Penstemon digitalis 'Husker Red'
- Beardtongue 'Prairie Dusk' Penstemon 'Prairie Dusk'
- Firecracker penstemon Penstemon eatonii
- Foxglove beardtongue Penstemon digitalis
- Hairy beardtongue Penstemon hirsutus
- Large beardtongue Penstemon grandiflorus
- Pineleaf penstemon Penstemon pinifolius
In these guides
Zone guides: Zone 4 · Zone 5 · Zone 6 · Zone 7 · Zone 8 · Zone 9
Every fact on record
- Type
- Perennial
- Size
- 2½ ft (70 cm) tall, 18 in (45 cm) wide
- Plant
- 16 in (40 cm) apart
- Height class
- Medium (24 in to 2½ ft)
- Place in the bed
- Middle
- Flowers
- Late May to early July; upright spires; blue
- Leaves
- Medium texture, semi-evergreen; green
- Shape
- Upright, clumping
- Hardy in
- USDA zones 4 to 9
- Light
- Full sun
- Soil
- Lean, dry and sharply drained
- Water
- Low
- Feeding
- None needed
Checked against 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.