When to Plant Petunia in Honolulu, Hawaii (96821)

Location

Honolulu, Hawaii

ZIP Code

96821

USDA Zone

12b

Last Frost

Thursday, January 1, 2026

📅 Your 2026 Planting Dates

Start Seeds Indoors

Thursday, October 23, 2025

Transplant Outdoors

Thursday, January 15, 2026

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

Pro tip: Start Petunia seeds indoors 10 weeks before your last frost date, then transplant outdoors around Thursday, January 15, 2026.

Add to Calendar (.ics)

🌿 Plant Details

Scientific Name Petunia × hybrida

Days to Maturity 75 days

Mature Height 6-18"

Spread 18"

Type Annual

☀️ Growing Requirements

☀️

Sun

Full Sun (6-8 hours)

💧

Water

Moderate (1 inch/week)

↔️

Spacing

12" apart

🌱

Planting Depth

0.13" deep

Seeds typically germinate in 7-14 days

Ready to Plant Petunia in Honolulu?

Get your seeds now so they're ready by Thursday, October 23, 2025.

🛒 Buy on Amazon 🌿 True Leaf Market

* Affiliate links — we may earn a small commission at no extra cost to you.

🤝 Good Companions

TomatoBasilMarigold

⚠️ Avoid Planting Near

Fennel

🤝

Full Companion Planting Guide for Petunia

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

🛠️ Supplies You'll Need

🌱 Seed Starting Trays 🪴 Seed Starting Mix 💡 Grow Lights 🌡️ Heat Mat 🌍 Flower Garden Soil ✂️ Pruning Shears 🚿 Watering Can

* Affiliate links — we may earn a small commission at no extra cost to you.

🌾 Seed Saving Guide

🟡 Moderate

⚠️ Many varieties are F1 hybrid — look for OP/heirloom types

When to Harvest Seed

Allow seed pods to form after flowers fade. Harvest when the small pod turns tan and begins to split.

Processing

Collect tiny seeds in a bowl when pod splits, dry 1 week before storing.

Seed Viability

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

Pro Tip

Most petunias are F1 hybrids — seedlings will not match the parent. Look for OP/heirloom varieties for reliable seed.

🪴

Raised Bed Planting Guides

A 4×8 bed fits 32 plants of Petunia. See plant counts, spacing grids, and companion pairings for every bed size →

Frequently Asked Questions

When should I plant Petunia in Honolulu, Hawaii? +

In Honolulu (Zone 12b), start Petunia seeds indoors around Thursday, October 23, 2025 and transplant outdoors around Thursday, January 15, 2026.

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

The average last spring frost date for Honolulu, Hawaii (ZIP 96821) is around Thursday, January 1, 2026. This is based on NOAA climate normals for USDA Zone 12b.

How long does Petunia take to grow? +

Petunia typically matures in about 75 days from transplanting. In Honolulu, that means you can expect harvest around Thursday, January 15, 2026 plus 75 days.

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

Petunia should be started indoors 10 weeks before your last frost date. In Honolulu, that means starting around Thursday, October 23, 2025.

📚 More Petunia Planting Guides

🗺️

Petunia in Hawaii

Zone-by-zone breakdown · Full state guide

→ 🌡️

Zone 12b Petunia Guide

All cities in your zone · Avg dates

→

Other Plants for Honolulu

Blackberry Blueberry Raspberry Strawberry Basil Chamomile Chives Cilantro Dill Lavender Lemon Balm Marjoram Mint Oregano Parsley Rosemary Sage Tarragon Thyme Acorn Squash Artichoke Arugula Asparagus Beefsteak Tomato Beet Bell Pepper Bok Choy Broccoli Brussels Sprout Butternut Squash Cabbage Cantaloupe Carrot Cauliflower Cayenne Pepper Celery Cherry Tomato Chili Pepper Collard Green Common Bean Cucumber Eggplant Endive Fennel Garlic Grape Tomato Green Bean Habanero Pepper Head Lettuce Heirloom Tomato Honeydew Melon Jalapeño Pepper Kale Kohlrabi Leaf Lettuce Leek Lima Bean Mustard Okra Onion Parsnip Pea Poblano Pepper Pole Pea Potato Pumpkin Radicchio Radish Rhubarb Roma Tomato Rutabaga Serrano Pepper Shallot Snap Pea Snow Pea Spaghetti Squash Spinach Squash Sweet Corn Sweet Potato Swiss Chard Tomato Turnip Watermelon Zucchini Sunflower Marigold Zinnia Nasturtium Cosmos Echinacea (Coneflower) Black-eyed Susan Apple Tree Pear Tree Peach Tree Plum Tree Sweet Cherry Tree Lemon Tree Borage Lemongrass Ginger Stevia Tomatillo Edamame Fava Bean Mâche Mizuna Sorrel Shishito Pepper Banana Pepper Watermelon Radish Napa Cabbage Popcorn Broccoli Raab Catnip Valerian Lemon Verbena Bee Balm Anise Lovage Chervil Summer Savory Dahlia Snapdragon Pansy Sweet Pea Morning Glory Hollyhock Larkspur Bachelor Button Columbine Impatiens Salvia (Annual) Sweet Alyssum Celosia Lupine Verbena Elderberry Grape Vine Red Currant Gooseberry Serviceberry Fig Tree Avocado Tree Persimmon Tree Olive Tree

Growing Petunia in Zone 12b

Petunia (Petunia × hybrida) is a popular garden vegetable. In Honolulu, Hawaii, which is in USDA Hardiness Zone 12b, you should start seeds indoors around Thursday, October 23, 2025.

Your average last frost date is Thursday, January 1, 2026. Plan to transplant your Petunia seedlings outdoors around Thursday, January 15, 2026, about 2 weeks after your last frost date.