Honolulu County, HI — Planting Guide
Your May planting checklist for Honolulu County, Hawaii
Each item below is timed to Honolulu County, Hawaii's frost dates and soil temperatures. Skip nothing, stress about nothing.
Honolulu County is in USDA Zone 12b. The average last spring frost is January 1 and the first fall frost is December 31, giving you a growing season of approximately 364 days.
At an elevation of 235 ft, Honolulu County receives approximately 44.3 in of rainfall annually. Summer highs average 88°F with winter lows around 65°F. The predominant soil type is Silty Clay.
🌡️ Zone
12b (°F to °F min)
❄️ Last Frost
January 1
🍂 First Frost
December 31
📅 Growing Season
364 days
⛰️ Elevation
235 ft
🌧️ Annual Rainfall
44.3 in
Monthly Watering Calendar
When you'll need to water your garden — based on average monthly rainfall vs. the ~1 inch/week most gardens need.
View detailed monthly data
| Month | Avg Rainfall | Rainy Days | Extra Water Needed | Watering Effort |
|---|---|---|---|---|
| Jan | 6.4 in | 11 days | — | Low |
| Feb | 4.7 in | 9 days | — | Low |
| Mar | 6.2 in | 11 days | — | Low |
| Apr | 3.9 in | 9 days | 0.4 in | Low |
| May | 2.5 in | 7 days | 1.8 in | High |
| Jun | 1.3 in | 5 days | 3 in | High |
| Jul | 0.9 in | 4 days | 3.4 in | Critical |
| Aug | 1.3 in | 4 days | 3 in | High |
| Sep | 2.2 in | 6 days | 2.1 in | High |
| Oct | 4.4 in | 10 days | — | Low |
| Nov | 4.2 in | 12 days | 0.1 in | Low |
| Dec | 6.2 in | 12 days | — | Low |
Annual total: 44.2 in. Water needs vary by crop — tomatoes need ~1.2"/week while herbs like rosemary need only 0.3"/week. Check individual plant pages for crop-specific water budgets that factor in your county's rainfall and soil drainage.
Honolulu County Soil Profile
Soil Type
Silty Clay
Soil pH
5.7-6.4
Drainage
Well Drained
Gardening Difficulty Score
Honolulu County is a very forgiving place to garden. Most plants thrive here with minimal effort.
Local Gardening Help in Honolulu County
Free expert help is closer than you think. Your county's cooperative extension office connects you with trained gardeners, soil testing labs, and local programs — all specific to Honolulu County's climate and soil.
County Extension Office
Honolulu County University of Hawaii CTAHR Extension Extension Office
Phone: 808-956-8397
Visit Extension Office Website →
Extension offices are run by land-grant universities and funded by the USDA. Their advice is free, research-based, and tailored to your county's specific conditions.
Master Gardener Program
Free gardening help from trained volunteers
Master Gardeners are community volunteers who complete 40–60 hours of university horticultural training. They answer gardening questions, diagnose plant problems, and offer workshops — all free.
Many extension offices run a Master Gardener hotline where you can call or email with photos of plant problems for free diagnosis.
Soil Testing
Available through your extension office
Before amending your soil, get it tested. Your extension office offers soil testing (typically $10–$25) that tells you exact pH, nutrient levels, and amendment recommendations specific to what you want to grow.
Services Available in Honolulu County
Finding local nurseries & garden centers in Honolulu County
Why Buy Local
Local nurseries carry plants that are proven to grow in your area. Staff can give you advice specific to Honolulu County's soil and climate that big-box stores can't. Plants from local growers are typically hardier because they're already acclimated to your zone.
How to Find Them
Search for "nurseries near Honolulu County HI" or "garden center Honolulu County" on Google Maps. Also check with your extension office — they often maintain lists of reputable local nurseries and plant sales.
Community gardens & gardening groups
Community gardens are a great way to learn from experienced gardeners in your area, especially if you're limited on space. Search "community garden Honolulu County HI" or check your extension office and local parks department. Facebook groups like "Honolulu County Gardeners" or "Hawaii Gardening" are also excellent for local advice and plant swaps.
Sunlight & Day Length
Monthly daylight hours and peak sun — critical for onion varieties, photoperiod-sensitive plants, and solar garden planning.
Longest Day
13.3 hours
Summer solstice daylight
Shortest Day
10.7 hours
Winter solstice daylight
Peak Sun Hours
9.2 hr/day peak (summer)
Peak sun hours (green dashed line below) account for cloud cover — this is the usable direct sunlight your garden actually receives. Most vegetables need 6+ peak sun hours.
Onion tip: Your shorter days favor short-day onion varieties like Vidalia, Texas 1015, and Red Creole. Plant in fall for best results.
View detailed monthly data
| Month | Daylight Hours | Peak Sun Hours | Day Length |
|---|---|---|---|
| January | 10.8 hr | 6.2 hr | Short day |
| February | 11.3 hr | 6.5 hr | Short day |
| March | 11.9 hr | 6.6 hr | Short day |
| April | 12.5 hr | 6.3 hr | Neutral |
| May | 13 hr | 7.8 hr | Neutral |
| June | 13.3 hr | 8.5 hr | Neutral |
| July | 13.2 hr | 9.2 hr | Neutral |
| August | 12.7 hr | 8.6 hr | Neutral |
| September | 12.1 hr | 7.5 hr | Neutral |
| October | 11.5 hr | 6.2 hr | Short day |
| November | 11 hr | 5.7 hr | Short day |
| December | 10.7 hr | 5.8 hr | Short day |
Peak sun hours factor in typical cloud cover — use these for solar panel and shade-planning calculations.
Soil Temperature & Composting Calendar
Monthly soil temps tell you when to plant warm-season crops, and when your compost pile is actively working.
Plant Warm Crops When
Soil reaches 60°F+
Soil warm enough from Jan through Dec.
Best Month to Compost
Jan
Microbial activity peaks when soil is warm.
Active Composting
12 months
Nearly year-round composting.
View detailed monthly data
| Month | Soil 4" Deep | Soil 8" Deep | Compost Activity | Time to Finish |
|---|---|---|---|---|
| Jan | 67°F | 70°F | 🔥 Peak | ~8 weeks |
| Feb | 66°F | 69°F | 🔥 Peak | ~8 weeks |
| Mar | 71°F | 71°F | 🔥 Peak | ~8 weeks |
| Apr | 77°F | 77°F | 🔥 Peak | ~8 weeks |
| May | 83°F | 82°F | 🔥 Peak | ~8 weeks |
| Jun | 90°F | 89°F | 🔥 Peak | ~8 weeks |
| Jul | 94°F | 92°F | 🔥 Peak | ~8 weeks |
| Aug | 96°F | 93°F | 🔥 Peak | ~8 weeks |
| Sep | 96°F | 92°F | 🔥 Peak | ~8 weeks |
| Oct | 87°F | 89°F | 🔥 Peak | ~8 weeks |
| Nov | 75°F | 82°F | 🔥 Peak | ~8 weeks |
| Dec | 69°F | 75°F | 🔥 Peak | ~8 weeks |
Highlighted rows = soil 60°F+ (safe for warm-season transplants). Compost finishes fastest during peak activity months.
Pest & Disease Pressure in Honolulu County
Computed from local climate patterns — warmer, humid conditions increase pest generations and fungal disease risk.
Insect Pest Pressure
High — expect multiple pest generations. Preventative measures essential.
Disease Risk
High fungal/bacterial risk. Space plants for airflow, water at soil level.
Seasonal Risk
View 6 common pests in your area
| Pest | Risk Level | Peak Months |
|---|---|---|
| Aphids | High | Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec |
| Whiteflies | High | Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec |
| Spider mites | High | Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov |
| Thrips | Moderate | Mar, Apr, May, Jun, Jul, Aug, Sep |
| Scale insects | Moderate | Apr, May, Jun, Jul, Aug, Sep, Oct |
| Nematodes | Low | May, Jun, Jul, Aug, Sep |
Organic pest management tips
- Install physical barriers: floating row covers, copper tape for slugs, mesh netting
- Apply Bt (Bacillus thuringiensis) for caterpillar control — safe for beneficial insects
- Use kaolin clay spray to deter a wide range of insects on fruiting crops
- Release beneficial insects: ladybugs for aphids, parasitic wasps for caterpillars
- Apply neem oil weekly during high-pressure months
- Mulch heavily (3-4 inches) to reduce soil-borne disease splash
Cover Crops for Honolulu County
Cover crops protect bare soil, fix nitrogen, suppress weeds, and improve soil structure — with county-specific planting dates.
Spring Cover Crops (1 options) — Build soil before the main growing season
| Crop | Plant By | Terminate | N-Fixing | Soil Benefit |
|---|---|---|---|---|
| Cowpeas (southern peas) | Jan 7 | Oct 29 | ✓ Yes | Excellent nitrogen fixer for warm climates, edible |
Wind & Microclimate
Wind dries soil, stresses plants, and affects frost patterns. Understanding your exposure helps with garden placement.
Seasonal Wind Speed
Spring: 14 mph Summer: 14 mph
Fall: 12 mph Winter: 11 mph
Prevailing wind: NE. Windy area — plant a windbreak hedge on the NE side of your garden.
Windbreak Benefit
7.3/10
Strongly recommended — a windbreak (fence, hedge, or row of tall crops like corn or sunflowers) will significantly improve garden yields.
Frost Pocket Risk
Moderate
Some terrain variation (743 ft range). Garden on slopes or higher ground if possible to avoid late-season frost pockets.
Rainwater Harvesting Potential
How much water you can collect, when to collect it, and what size system you need for your garden.
Annual Collection
22,029 gal
Per 1,000 sq ft of roof area (at 80% collection efficiency)
Recommended Setup
6 rain barrels (55 gal each)
For a typical 500 sq ft garden. Serious collectors: consider a 1,000 gal tank.
Legal Status
Unrestricted
Rainwater harvesting is fully legal in your state with no restrictions.
Best Collection Months
Jan, Feb, Mar, Dec
Highest rainfall months — your barrels will fill up quickly during these months.
Months to Draw From Storage
Jul
Dry months when you'll rely on stored water — size your storage for this gap.
Rainwater collection tips for your area
- Your county receives approximately 44.2 inches of rain per year
- A 1,000 sq ft roof can collect roughly 22,029 gallons annually
- Rainwater harvesting is fully legal in your state
- Use a first-flush diverter to keep roof debris out of your collection
Soil & Growing Conditions in Honolulu County
Soil Type
Silty Clay
Soil pH 5.7–6.4 · Well Drained drainage
Native soil is well-suited to most vegetables and herbs with regular compost additions.
Watering Needs
Drought stress: 5.5/10
Moderate drought pressure. Drip irrigation and mulching are highly recommended to maintain soil moisture through summer.
Season Tips
364-day frost-free season
Your long season supports multiple successions and heat-demanding crops like melons, sweet potatoes, and peppers. Plant warm-season crops as soon as soil warms.
Your Free Printable Garden Planner
Plan every bed, every planting, every harvest — in one place. This 24-page printable includes your zone's planting calendar, a month-by-month task list, a seed inventory tracker, a harvest log, and succession-planting charts. Built to print, write in, and actually use all season.
Recommended for Your Garden
Reduce heat stress and sun scorch in hot climates with UV-stabilized shade cloth.
Cedar raised bed kit — ideal for poor soil, clay, or small-space gardening.
Improve drainage and aeration in heavy clay soils with horticultural perlite.
🥬 Vegetables to Grow in Honolulu County
4 vegetables that grow well in Zone 12b with planting dates for Honolulu County.
Show all 4 vegetables with planting dates
| Plant | Start Indoors | Direct Sow | Transplant | Harvest | Days to Maturity |
|---|---|---|---|---|---|
| Ginger | Nov 20 | Jan 1 | Jan 8 | Sep 10 – Nov 19 | 240–300 |
| Microgreens | Dec 4 | Dec 11 | Jan 1 | Jan 8 – Feb 5 | 7–21 |
| Turmeric | Nov 20 | Jan 1 | Jan 8 | Sep 10 – Nov 19 | 240–300 |
| Yam | Nov 20 | Jan 1 | Jan 8 | Jul 9 – Dec 24 | 180–330 |
Monthly Planting Guide for Honolulu County
Gardening Guides & Resources
Helpful guides from The Ultimate Homestead to improve your garden in Honolulu County.
Frequently Asked Questions
What planting zone is Honolulu County, HI?
Honolulu County is in USDA Hardiness Zone 12b. This zone classification determines which perennial plants survive winter and sets the baseline for frost timing across the county.
When is the last frost in Honolulu County, HI?
Based on 30 years of NOAA weather station data, the median last spring frost in Honolulu County falls around January 1. Plan transplants and direct-sow dates relative to this date.
When is the first fall frost in Honolulu County, HI?
The median first fall frost in Honolulu County arrives around December 31. Harvest or protect frost-sensitive crops — tomatoes, peppers, basil, squash — before this date.
What is the soil like in Honolulu County for gardening?
Honolulu County has predominantly Silty Clay soil with a pH range of 5.7–6.4 and Well Drained drainage. Most vegetables and herbs grow well with standard composting and seasonal soil amendment.
What is grown commercially in Honolulu County?
Honolulu County has commercial agriculture that includes Macadamia Nuts, Coffee, Cattle. These crops reflect the local climate and soil conditions — what succeeds commercially often translates well to home gardens in the same area.
Is Honolulu County a good location for home gardening?
Honolulu County scores 86/100 (Excellent) on our Microclimate Index, which combines frost reliability, drought pressure, soil challenge, elevation risk, and long-term climate trend. This is an above-average location for home gardening with relatively predictable growing conditions.
Your Honolulu County Garden Planner — Free
A 24-page printable planner built for Honolulu County (Zone 12b). Planting dates, a month-by-month schedule, harvest log, seed inventory, and succession charts — all dialed in for your exact growing season.
The Gardener's Encyclopedia to Companion Planting
The pairings that make vegetables, herbs, and flowers grow better — and the ones that quietly wreck a bed.
- Proven pairings for 200+ vegetables, herbs, flowers, and fruits
- Full seed-starting + planting schedule with timing and spacing
- Bonus: square-foot gardening guide + printable seasonal planners
Seed Saving & Storage Guide
Most saved seeds go bad before next season. This shows exactly when to pick, how to dry, and where to store seeds from 200 plants so yours don't.
- 200 plants, step-by-step: life cycle, pollination type, isolation
- Exact temperature + humidity ranges that keep seeds viable
- Bonus: searchable Google Sheets tracker + custom GPT assistant
Composting Guide for Homesteaders
Turn kitchen scraps and yard waste into compost that actually feeds the garden — instead of a pile that smells, attracts pests, and never breaks down.
- 14 sections on composting methods, soil science, and troubleshooting
- The 7-step hot-compost system from start to finish
- Bonus tools: troubleshooting chart, safety guide, monitoring log