When to Plant Cosmos in Mount Wilson, California (91023)

Location

Mount Wilson, California

ZIP Code

91023

USDA Zone

Last Frost

Friday, February 20, 2026

📅 Your 2026 Planting Dates

Direct Sow Outdoors

Friday, February 20, 2026

Based on last frost Friday, February 20, 2026 · Zone 9a

Pro tip: Sow Cosmos directly outdoors around your last frost date when soil starts to warm.

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🌿 Plant Details

Scientific Name Cosmos bipinnatus

Days to Maturity 60 days

Mature Height 24-60"

Spread 18"

Type Annual

☀️ Growing Requirements

☀️

Sun

Full Sun (6-8 hours)

💧

Water

Low-Moderate

↔️

Spacing

18" apart

🌱

Planting Depth

0.25" deep

Seeds typically germinate in 7-14 days

Ready to Plant Cosmos in Mount Wilson?

Get your seeds now so they're ready by Friday, February 20, 2026.

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🤝 Good Companions

CornSquashTomato

🤝

Full Companion Planting Guide for Cosmos

See which plants help Cosmos thrive, which to keep away, and raised bed layout tips →

🛠️ Supplies You'll Need

🌍 Flower Garden Soil ✂️ Pruning Shears 🚿 Watering Can

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🌾 Seed Saving Guide

🟢 Easy

✅ Open-pollinated — seeds grow true to parent

When to Harvest Seed

Harvest narrow seed heads when flower petals drop and the base turns brown and dry.

Processing

Pull seeds directly from dried flower heads, dry 1 week in an open envelope.

Seed Viability

Up to 3 years when stored cool, dark, and dry.

Pro Tip

Cosmos cross freely within the species — isolate by 150 feet from other cosmos for pure color.

🪴

Raised Bed Planting Guides

A 4×8 bed fits 14 plants of Cosmos. See plant counts, spacing grids, and companion pairings for every bed size →

Frequently Asked Questions

When should I plant Cosmos in Mount Wilson, California? +

In Mount Wilson (Zone 9a), direct sow Cosmos outdoors around Friday, February 20, 2026.

What is the last frost date for Mount Wilson, California? +

The average last spring frost date for Mount Wilson, California (ZIP 91023) is around Friday, February 20, 2026. This is based on NOAA climate normals for USDA Zone 9a.

How long does Cosmos take to grow? +

Cosmos typically matures in about 60 days from direct sowing. In Mount Wilson, that means you can expect harvest around Friday, February 20, 2026 plus 60 days.

Should I start Cosmos indoors or direct sow in California? +

Cosmos does best when direct sown outdoors. In Mount Wilson, sow directly around Friday, February 20, 2026.

📚 More Cosmos Planting Guides

🗺️

Cosmos in California

Zone-by-zone breakdown · Full state guide

→ 🌡️

Zone 9a Cosmos Guide

All cities in your zone · Avg dates

→

Other Plants for Mount Wilson

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Growing Cosmos in Zone 9a

Cosmos (Cosmos bipinnatus) is a popular garden vegetable. In Mount Wilson, California, which is in USDA Hardiness Zone 9a, you should direct sow seeds around Friday, February 20, 2026.

Your average last frost date is Friday, February 20, 2026. Cosmos grows well when direct sown, as it doesn't transplant well or germinates quickly outdoors.