When to plant Astilbe in Douglas County, CO
Plant Astilbe in Douglas County during the brief June 5–June 19 window. With 121 frost-free days, fall plantings can't mature before September 20.
When to Plant Astilbe in Douglas County, CO
Astilbe (Astilbe x arendsii) is a premier perennial for shaded, moist gardens, producing feathery plume-like flower spikes in shades of pink, red, white, and lavender above deeply cut, ferny foliage. Originating in Asia and naturalized in shade gardens worldwide, astilbe thrives where many sun-lovers fail: under trees, beside water features, or in persistently moist woodland borders. The dried flower plumes remain attractive through fall and winter. Different cultivars extend the bloom season from early summer through early fall when several varieties are combined.
Douglas County, Colorado is in USDA Zone 5b. The average last spring frost is May 22 and the first fall frost is September 20, giving you a growing season of approximately 121 days.
At an elevation of 7,421 feet, Douglas County receives approximately 12.9 inches of rainfall annually with predominantly loam soil. Summer highs average 85°F, providing good warmth for Astilbe during the growing season. With low rainfall, drip irrigation is essential for growing Astilbe successfully. Mulch heavily to conserve soil moisture.
Douglas County Soil Profile
Soil Type
Loam
Soil pH
6.7-8.4
Drainage
Well Drained
Monthly Watering Guide for Astilbe
Astilbe needs approximately 1 inches of water per week (4.3" per month). Here's how your county's rainfall compares month by month.
| Month | Astilbe Needs | Rainfall | You Supplement | Action |
|---|---|---|---|---|
| Jan | — | 1" | 0" | ❄️ Dormant |
| Feb | — | 0.8" | 0" | ❄️ Dormant |
| Mar | — | 1.1" | 0" | ❄️ Dormant |
| Apr | — | 1.3" | 0" | ❄️ Dormant |
| May | 4.3" | 1.4" | 2.9" | 🚿 Regular watering |
| Jun | 4.3" | 1" | 3.3" | 🚿 Regular watering |
| Jul | 4.3" | 1" | 3.3" | 🚿 Regular watering |
| Aug | 4.3" | 1.4" | 2.9" | 🚿 Regular watering |
| Sep | 4.3" | 1.1" | 3.2" | 🚿 Regular watering |
| Oct | — | 1.1" | 0" | ❄️ Dormant |
| Nov | — | 1" | 0" | ❄️ Dormant |
| Dec | — | 0.8" | 0" | ❄️ Dormant |
Water needs are for active growing months only (May–Sep in Douglas County). Supplement amounts are based on average rainfall — actual needs vary with temperature, soil, and mulching.
Astilbe Planting Timeline — Douglas County, CO
Astilbe Planting Calendar
| Activity | When | Date Range |
|---|---|---|
| Start Indoors | March 20 | Mar 20 – Apr 3 |
| Transplant Outdoors | June 5 | Jun 5 – Jun 19 |
| Bloom | August 21 | Aug 21 – Oct 30 |
Plant 1.5" deep · 18" apart · Rows 24" apart
Month-by-Month Timeline
| Month | Activities |
|---|---|
| January | — |
| February | — |
| March | Start Indoors |
| April | Start Indoors |
| May | — |
| June | Transplant Outdoors |
| July | — |
| August | Bloom |
| September | Bloom |
| October | Bloom |
| November | — |
| December | — |
Growing Conditions
☀️ Sun
Partial Shade (3-6 hours)
💧 Water
High — keep soil consistently moist
📅 Days to Maturity
70–100 days
🧪 Soil pH
Needs 6–8 · Your soil: N/A
🗺️ USDA Zone
Zone 5b
📆 Growing Season
121 days in Douglas County
Growing Tips for Douglas County
Plant bare-root crowns in spring as soon as the ground is workable, setting eyes 1–2 inches below soil level. Fall planting (Zones 4+) is equally effective. Consistent moisture is non-negotiable — astilbe will wilt and scorch in drought. Amend with generous compost to improve moisture retention. Partial shade (morning sun, afternoon shade) is ideal; deep shade reduces bloom but foliage remains. Fertilize lightly in spring with a balanced slow-release granular. Divide every 3–5 years in early spring as clumps become congested. Leave plumes standing through winter for ornamental interest and wildlife value. Year 2+ plants produce the fullest bloom spikes; first-year crowns may flower lightly or not at all.
Companion Planting
Good Companions
Check more plant combinations with our Companion Planting Checker →
Astilbe in Other Locations
Frequently Asked Questions
When should I plant Astilbe in Douglas County, CO?
Douglas County is in Zone 5b with an average last frost of May 22. Plan your Astilbe planting based on this frost date — see the calendar above for exact timing.
What planting zone is Douglas County, CO?
Douglas County, Colorado is in USDA Hardiness Zone 5b. The average last spring frost is May 22 and first fall frost is September 20.
When should I plant Astilbe in Douglas County, CO?
In Douglas County, CO, plant Astilbe after the last frost (around May 22) and before the first frost (around September 20). Best results when soil holds above 50°F.
What growing zone is Douglas County, CO for Astilbe?
Douglas County sits in USDA Zone 5b. Astilbe grows reliably in zones 3a through 8b, so it's a good fit here.
Can Astilbe grow in Douglas County's climate?
Yes — Astilbe grows well in Douglas County's temperate climate. Douglas County averages a 121-day frost-free season, with last frost around May 22 and first frost around September 20.
Your Douglas County Garden Planner — Free
A 22-page printable planner built for Douglas County (Zone 5b). Planting dates, a month-by-month schedule, harvest log, seed inventory, and succession charts — all dialed in for your exact growing season.