When to plant Astilbe in Elmore County, ID
Elmore County sits in cold Zone 7a. Plant Astilbe May 4–May 18 for the single annual harvest; the October 14 first frost closes the window.
When to Plant Astilbe in Elmore County, ID
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.
Elmore County, Idaho is in USDA Zone 7a. The average last spring frost is April 27 and the first fall frost is October 14, giving you a growing season of approximately 170 days.
At an elevation of 5,944 feet, Elmore County receives approximately 20.3 inches of rainfall annually with predominantly loam soil. Summer highs average 88°F, providing good warmth for Astilbe during the growing season.
Elmore County Soil Profile
Soil Type
Loam
Soil pH
6.7-8.3
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 | — | 1.6" | 0" | ❄️ Dormant |
| Feb | — | 1.3" | 0" | ❄️ Dormant |
| Mar | — | 1.7" | 0" | ❄️ Dormant |
| Apr | 4.3" | 1.9" | 2.4" | 🚿 Regular watering |
| May | 4.3" | 2.7" | 1.6" | 💧 Light watering |
| Jun | 4.3" | 1.6" | 2.7" | 🚿 Regular watering |
| Jul | 4.3" | 1.6" | 2.7" | 🚿 Regular watering |
| Aug | 4.3" | 1.9" | 2.4" | 🚿 Regular watering |
| Sep | 4.3" | 1.6" | 2.7" | 🚿 Regular watering |
| Oct | 4.3" | 1.7" | 2.6" | 🚿 Regular watering |
| Nov | — | 1.4" | 0" | ❄️ Dormant |
| Dec | — | 1.2" | 0" | ❄️ Dormant |
Water needs are for active growing months only (Apr–Oct in Elmore County). Supplement amounts are based on average rainfall — actual needs vary with temperature, soil, and mulching.
Astilbe Planting Timeline — Elmore County, ID
Astilbe Planting Calendar
| Activity | When | Date Range |
|---|---|---|
| Start Indoors | February 23 | Feb 23 – Mar 9 |
| Transplant Outdoors | May 4 | May 4 – May 18 |
| Bloom | July 13 | Jul 13 – Sep 7 |
Plant 1.5" deep · 18" apart · Rows 24" apart
Month-by-Month Timeline
| Month | Activities |
|---|---|
| January | — |
| February | Start Indoors |
| March | Start Indoors |
| April | — |
| May | Transplant Outdoors |
| June | — |
| July | Bloom |
| August | Bloom |
| September | Bloom |
| October | — |
| 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 7a
📆 Growing Season
170 days in Elmore County
Growing Tips for Elmore 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 Elmore County, ID?
Elmore County is in Zone 7a with an average last frost of April 27. Plan your Astilbe planting based on this frost date — see the calendar above for exact timing.
What planting zone is Elmore County, ID?
Elmore County, Idaho is in USDA Hardiness Zone 7a. The average last spring frost is April 27 and first fall frost is October 14.
When should I plant Astilbe in Elmore County, ID?
In Elmore County, ID, plant Astilbe after the last frost (around April 27) and before the first frost (around October 14). Best results when soil holds above 50°F.
What growing zone is Elmore County, ID for Astilbe?
Elmore County sits in USDA Zone 7a. Astilbe grows reliably in zones 3a through 8b, so it's a good fit here.
Can Astilbe grow in Elmore County's climate?
Yes — Astilbe grows well in Elmore County's temperate climate. Elmore County averages a 170-day frost-free season, with last frost around April 27 and first frost around October 14.
Your Elmore County Garden Planner — Free
A 22-page printable planner built for Elmore County (Zone 7a). Planting dates, a month-by-month schedule, harvest log, seed inventory, and succession charts — all dialed in for your exact growing season.