When to Plant Cosmos in Honolulu, Hawaii (96813)

Location

Honolulu, Hawaii

ZIP Code

96813

USDA Zone

12b

Last Frost

Thursday, January 1, 2026

📅 Your 2026 Planting Dates

Direct Sow Outdoors

Thursday, January 1, 2026

Based on last frost Thursday, January 1, 2026 · Zone 12b

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 Honolulu?

Get your seeds now so they're ready by Thursday, January 1, 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 Honolulu, Hawaii? +

In Honolulu (Zone 12b), direct sow Cosmos outdoors around Thursday, January 1, 2026.

What is the last frost date for Honolulu, Hawaii? +

The average last spring frost date for Honolulu, Hawaii (ZIP 96813) is around Thursday, January 1, 2026. This is based on NOAA climate normals for USDA Zone 12b.

How long does Cosmos take to grow? +

Cosmos typically matures in about 60 days from direct sowing. In Honolulu, that means you can expect harvest around Thursday, January 1, 2026 plus 60 days.

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

Cosmos does best when direct sown outdoors. In Honolulu, sow directly around Thursday, January 1, 2026.

📚 More Cosmos Planting Guides

🗺️

Cosmos in Hawaii

Zone-by-zone breakdown · Full state guide

→ 🌡️

Zone 12b Cosmos Guide

All cities in your zone · Avg dates

→

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Growing Cosmos in Zone 12b

Cosmos (Cosmos bipinnatus) is a popular garden vegetable. In Honolulu, Hawaii, which is in USDA Hardiness Zone 12b, you should direct sow seeds around Thursday, January 1, 2026.

Your average last frost date is Thursday, January 1, 2026. Cosmos grows well when direct sown, as it doesn't transplant well or germinates quickly outdoors.