Astilbe in Zone 4a & 4b
Astilbe x arendsii
Yes, astilbe grows in zones 4a and 4b and comes back each year. Plant it outside May 1–May 8.
When to plant astilbe in zone 4b
Zones 4a and 4b share the same average frost dates, so this page covers both.
Method: Direct sow: Direct-sow outside. It doesn't transplant well, or doesn't need the extra time indoors.
| Start indoors | Not typically started indoors |
|---|---|
| Direct sow / transplant outside | May 1–May 8 |
| Estimated bloom | First real harvest the season after planting |
Spacing: about 2 square feet per plant (square-foot-gardening spacing). Mature height: 1.5-4.5 ft.
Before you plant: Astilbe has a winter-hardiness note for zone 4b. See the details below.
Plant alongside it in zone 4b
These go outside within two weeks of astilbe in zone 4b, so one planting day can cover them.
- Agrostemma (Corn Cockle)May 1–May 8
- Bachelor's Button (Cornflower)May 1–May 8
- Basket-of-GoldMay 1–May 8
- Bupleurum (Thoroughwax)May 1–May 8
- CalendulaMay 1–May 8
- California PoppyMay 1–May 8
Astilbe gives feathery plumes in shade, which is a rare combination. It asks for one thing in return and it does not negotiate.
How astilbe fits zone 4's season
Zone 4b's frost-free window runs about 128 days, from May 15 to September 20. Astilbe takes about 1 growing season to reach full flowering, so what matters here is how many of these seasons it gets, not whether one is long enough.
Astilbe is hardy in zones 3-9, so in zone 4b it stays in the ground through winter and comes back rather than being replanted. Allow about 1.5 ft of spread at maturity.
Astilbe planting windows in every zone
Zone 4b is highlighted. Useful for comparing against a neighbouring zone, or for a garden somewhere else.
| Zone | Start indoors | Sow or plant out | Bloom |
|---|---|---|---|
| 3a & 3b | Not usually | May 16–May 23 | First real harvest the season after planting |
| 4a & 4b (your zone) | Not usually | May 1–May 8 | First real harvest the season after planting |
| 5a & 5b | Not usually | Apr 26–May 3 | First real harvest the season after planting |
| 6a & 6b | Not usually | Apr 11–Apr 18 | First real harvest the season after planting |
| 7a & 7b | Not usually | Mar 27–Apr 3 | First real harvest the season after planting |
| 8a & 8b | Not usually | Mar 11–Mar 18 | First real harvest the season after planting |
| 9a & 9b | Not usually | Feb 1–Feb 8 | First real harvest the season after planting |
Good companions
No well-established companions for astilbe.
Where to buy astilbe seeds
True Leaf Market (each supplier's astilbe seeds)
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See it growing
Search YouTube for "growing astilbe" videos → (a live search, not a hand-picked video; results vary and aren't vetted by us)
← Back to the full zone 4b calendar · Plan your beds in the Garden Designer →
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Growing Astilbe in USDA zone 4b: when to start seeds indoors, when to direct-sow, and when to expect a bloom.
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