Multnomah County, OR — Planting Guide
Multnomah County is in USDA Zone 9a. The average last spring frost is April 14 and the first fall frost is October 27, giving you a growing season of approximately 196 days.
At an elevation of 357 ft, Multnomah County receives approximately 35.9 in of rainfall annually. Summer highs average 84°F with winter lows around 47°F. The predominant soil type is Silt Loam.
Based on 31 years of NOAA climate station data, the last frost date here varies by 45 days year to year — ranging from March 25 in warm years to May 9 in cold years. The growing season is trending longer by about 1.6 days per decade. Multnomah County scores 67/100 (Good) on the Microclimate Index.
🌡️ Zone
9a (20°F to 25°F min)
❄️ Last Frost
April 14
🍂 First Frost
October 27
📅 Growing Season
196 days
⛰️ Elevation
357 ft
🌧️ Annual Rainfall
35.9 in
Monthly Watering Calendar for Multnomah County
When you'll need to water your garden — based on average monthly rainfall vs. the ~1 inch/week most gardens need.
Quick context: The 1-inch-per-week rule applies to most vegetable crops. Multnomah County averages 36" a year — divide by 52 and compare to that 1" target. Some months are above, some below; that's where the calendar earns its keep.
View detailed monthly data
| Month | Avg Rainfall | Rainy Days | Extra Water Needed | Watering Effort |
|---|---|---|---|---|
| Jan | 5.8 in | 16 days | — | None |
| Feb | 4.1 in | 13 days | 0.2 in | Low |
| Mar | 3.5 in | 15 days | 0.8 in | Moderate |
| Apr | 2.3 in | 14 days | 2 in | High |
| May | 2.3 in | 11 days | 2 in | High |
| Jun | 1.3 in | 7 days | 3 in | High |
| Jul | 0.6 in | 3 days | 3.7 in | Critical |
| Aug | 0.7 in | 2 days | 3.6 in | Critical |
| Sep | 1.6 in | 4 days | 2.7 in | High |
| Oct | 2.9 in | 9 days | 1.4 in | Moderate |
| Nov | 6 in | 17 days | — | Low |
| Dec | 4.9 in | 17 days | — | None |
Annual total: 36 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.
Multnomah County Soil Profile
Soil Type
Silt Loam
Soil pH
5.8-6.3
Drainage
Well Drained
Frost Risk Probability
Based on 31 years of NOAA weather station data from 3 stations
Beginners: Plant frost-sensitive crops (tomatoes, peppers, squash) after the "Safe" date on the left. Harvest or cover them before the "Protect by" date on the right. Hardy crops (lettuce, peas, kale) can go in the yellow transition zones.
How to read this table: "Conservative" means you're safe from frost 9 out of 10 years — best for beginners and frost-sensitive crops. "Average year" is the typical date. "Aggressive" means only 1 in 10 years is that warm — experienced gardeners with frost protection can try these dates.
| Planting Strategy | Last Spring Frost | First Fall Frost | Frost-Free Days |
|---|---|---|---|
| Conservative (safest) | May 9 | Nov 19 | 194 days |
| Cautious | Apr 22 | Nov 6 | 198 days |
| Average year | Apr 14 | Oct 27 | 196 days |
| Optimistic | Apr 8 | Oct 18 | 193 days |
| Aggressive (risky) | Mar 25 | Oct 9 | 198 days |
Not very — frost dates can vary by ±45 days year-to-year. Use the "Conservative" row in the table below, and keep row covers handy for surprise late frosts.
Yes — growing seasons are getting longer here (about 1.6 days per decade). Spring is arriving earlier than it used to. Good news for gardeners.
Gardening Difficulty Score
Multnomah County offers good growing conditions. A little planning around frost dates goes a long way.
Local Gardening Help in Multnomah 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 Multnomah County's climate and soil.
County Extension Office
Multnomah County Oregon State University Extension Extension Office
Phone: 541-737-2713
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 Multnomah County
Finding local nurseries & garden centers in Multnomah County
Why Buy Local
Local nurseries carry plants that are proven to grow in your area. Staff can give you advice specific to Multnomah 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 Multnomah County OR" or "garden center Multnomah 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 Multnomah County OR" or check your extension office and local parks department. Facebook groups like "Multnomah County Gardeners" or "Oregon Gardening" are also excellent for local advice and plant swaps.
What to Plant After Your Harvest
After your first crops finish, use the remaining frost-free days to grow a second round.
Show 6 more succession options
Sunlight & Day Length in Multnomah County
Monthly daylight hours and peak sun — critical for onion varieties, photoperiod-sensitive plants, and solar garden planning.
Why it matters: Onions are a great example of why day length matters. They "bulb up" only when daylight hits a specific number of hours — plant the wrong variety (short-day in the north, long-day in the south) and you'll get tiny bulbs no matter how well you grow them. Multnomah County's latitude determines which onion varieties succeed.
Longest Day
15.5 hours
Summer solstice daylight
Shortest Day
8.5 hours
Winter solstice daylight
Peak Sun Hours
10.2 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 long summer days (14+ hours) support long-day onion varieties like Walla Walla, Sweet Spanish, and Ailsa Craig.
View detailed monthly data
| Month | Daylight Hours | Peak Sun Hours | Day Length |
|---|---|---|---|
| January | 8.9 hr | 2.3 hr | Short day |
| February | 10.1 hr | 2.9 hr | Short day |
| March | 11.6 hr | 4.1 hr | Short day |
| April | 13.3 hr | 5.4 hr | Neutral |
| May | 14.7 hr | 7.4 hr | Long day |
| June | 15.5 hr | 8.4 hr | Long day |
| July | 15.2 hr | 10.2 hr | Long day |
| August | 13.9 hr | 9.8 hr | Neutral |
| September | 12.3 hr | 7.4 hr | Neutral |
| October | 10.7 hr | 4.3 hr | Short day |
| November | 9.2 hr | 2.2 hr | Short day |
| December | 8.5 hr | 1.9 hr | Short day |
Peak sun hours factor in typical cloud cover — use these for solar panel and shade-planning calculations.
Soil Temperature & Composting in Multnomah County
Monthly soil temps tell you when to plant warm-season crops, and when your compost pile is actively working.
Quick context: Soil heats slower than air in spring and cools slower in fall. That's why "warm" April air doesn't mean "plant tomatoes" — soil still trails by weeks. Multnomah County's monthly soil curve makes the lag visible.
Plant Warm Crops When
Soil reaches 60°F+
Soil warm enough from May through Nov.
Best Month to Compost
May
Microbial activity peaks when soil is warm.
Active Composting
9 months
Nearly year-round composting.
View detailed monthly data
| Month | Soil 4" Deep | Soil 8" Deep | Compost Activity | Time to Finish |
|---|---|---|---|---|
| Jan | 46°F | 50°F | 🐢 Slow | ~24 weeks |
| Feb | 45°F | 49°F | 🐢 Slow | ~24 weeks |
| Mar | 51°F | 53°F | ♻️ Active | ~14 weeks |
| Apr | 59°F | 62°F | ♻️ Active | ~14 weeks |
| May | 72°F | 70°F | 🔥 Peak | ~8 weeks |
| Jun | 80°F | 77°F | 🔥 Peak | ~8 weeks |
| Jul | 89°F | 84°F | 🔥 Peak | ~8 weeks |
| Aug | 91°F | 85°F | 🔥 Peak | ~8 weeks |
| Sep | 86°F | 82°F | 🔥 Peak | ~8 weeks |
| Oct | 72°F | 73°F | 🔥 Peak | ~8 weeks |
| Nov | 61°F | 65°F | ♻️ Active | ~14 weeks |
| Dec | 47°F | 55°F | 🐢 Slow | ~24 weeks |
Highlighted rows = soil 60°F+ (safe for warm-season transplants). Compost finishes fastest during peak activity months.
Pest & Disease Pressure in Multnomah County
Computed from local climate patterns — warmer, humid conditions increase pest generations and fungal disease risk.
The practical takeaway: The most successful gardeners in high-pressure regions don't spray more — they design around the problem. Crop rotation, companion planting, and resistant varieties beat reactive spraying.
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 | Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov |
| Squash vine borers | Moderate | May, Jun, Jul |
| Stink bugs | High | Apr, May, Jun, Jul, Aug, Sep, Oct |
| Whiteflies | Moderate | May, Jun, Jul, Aug, Sep, Oct |
| Spider mites | Moderate | Jun, Jul, Aug, Sep |
| Fire ants | Moderate | Mar, Apr, May, Jun, Jul, Aug, Sep, Oct |
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 Multnomah County
Cover crops protect bare soil, fix nitrogen, suppress weeds, and improve soil structure — with planting dates calibrated for your area.
Why this matters: The "chop and drop" approach to cover crops: cut them down right before flowering, let them lay on the surface as mulch, plant your vegetables through the mulch. Less work, healthier soil.
Spring Cover Crops (4 options) — Build soil before the main growing season
| Crop | Plant By | Terminate | N-Fixing | Soil Benefit |
|---|---|---|---|---|
| Buckwheat | Apr 19 | Aug 18 | — | Rapid growth, attracts pollinators, suppresses weeds |
| Cowpeas (southern peas) | Apr 21 | Sep 1 | ✓ Yes | Excellent nitrogen fixer for warm climates, edible |
| Sorghum-sudan grass | Apr 21 | Aug 25 | — | Massive biomass, breaks compaction, suppresses nematodes |
| White clover | Mar 14 | Sep 1 | ✓ Yes | Living mulch, fixes nitrogen, permanent ground cover |
Summer Cover Crops (1 options) — Fill gaps and suppress weeds between plantings
| Crop | Plant By | Terminate | N-Fixing | Soil Benefit |
|---|---|---|---|---|
| Sunflowers | May 5 | Sep 29 | — | Deep roots break compaction, attract pollinators and beneficial insects |
Fall Cover Crops (7 options) — Plant after harvest to protect soil over winter
| Crop | Plant By | Terminate | N-Fixing | Soil Benefit |
|---|---|---|---|---|
| Austrian winter peas | Aug 19 | Mar 31 | ✓ Yes | Fixes nitrogen, good for heavy clay soils |
| Crimson clover | Aug 31 | Mar 31 | ✓ Yes | Fixes nitrogen, attracts pollinators in spring |
| Daikon radish | Aug 30 | Mar 31 | — | Deep taproot breaks compaction, excellent for clay soils |
| Hairy vetch | Aug 17 | Mar 24 | ✓ Yes | Excellent nitrogen fixer, good for depleted soils |
| Oats | Sep 23 | Mar 31 | — | Quick biomass, winterkills in cold zones — no spring tillage needed |
| Winter rye | Jul 24 | Mar 31 | — | Suppresses weeds, prevents erosion, breaks up compacted soil |
| Winter wheat | Aug 7 | Mar 24 | — | Erosion control, weed suppression, good biomass |
Wind & Microclimate in Multnomah County
What this means for you: A 10 mph wind doesn't feel like much, but it triples leaf transpiration vs. still air. Multnomah County's 8.4 mph average means most days are gentle on plants, but consider how a 20+ mph spring gust would affect a flat of seedlings hardened off too quickly.
Wind dries soil, stresses plants, and affects frost patterns. Understanding your exposure helps with garden placement.
Seasonal Wind Speed
Spring: 14 mph Summer: 8 mph
Fall: 10 mph Winter: 12 mph
Prevailing wind: SW. Moderate wind — consider a temporary windbreak for young seedlings.
Windbreak Benefit
5/10
Moderately beneficial — a simple fence or trellis can protect delicate crops from wind stress.
Frost Pocket Risk
Moderate
Some terrain variation (480 ft range). Garden on slopes or higher ground if possible to avoid late-season frost pockets.
Rainwater Harvesting in Multnomah County
How much water you can collect, when to collect it, and what size system you need for your garden.
For new gardeners: Most gardens use 0.5-1 gallon per square foot per week in summer. Multnomah County's 36" annual rainfall is enough to cover most needs if you can capture it. Rain barrels under downspouts are the simplest entry point.
Annual Collection
17,942 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 2,250 gal tank.
Legal Status
Unrestricted
Rainwater harvesting is fully legal in your state with no restrictions.
Best Collection Months
Jan, Feb, Nov, Dec
Highest rainfall months — your barrels will fill up quickly during these months.
Months to Draw From Storage
Jul, Aug
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 36.0 inches of rain per year
- A 1,000 sq ft roof can collect roughly 17,942 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 Multnomah County
Soil Type
Silt Loam
Soil pH 5.8–6.3 · Excessively Drained drainage
Native soil is well-suited to most vegetables and herbs with regular compost additions.
Watering Needs
Drought stress: 3.5/10
Low-to-moderate drought stress. Plan to water 1–2 times per week during peak summer. (35.9 in. annual rainfall)
Season Tips
196-day frost-free season
Plenty of time for warm-season crops. Start heat-lovers indoors 6–8 weeks before last frost to maximise your harvest window.
Your Free Printable Garden Planner
Plan every bed, every planting, every harvest — in one place. This 22-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.
Test your soil pH, nitrogen, phosphorus, and potassium levels before planting.
Get instant, accurate soil pH readings to fine-tune your amendments.
Monthly Planting Guide for Multnomah County
Gardening Guides & Resources
Helpful guides from The Ultimate Homestead to improve your garden in Multnomah County.
Frequently Asked Questions
What planting zone is Multnomah County, OR?
Multnomah County is in USDA Hardiness Zone 9a. 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 Multnomah County, OR?
Based on 31 years of NOAA weather station data, the median last spring frost in Multnomah County falls around April 14. In 8 out of 10 years, last frost lands between March 25 and May 9 — a 45-day window of variability. Use May 9 as your conservative safe-to-plant date for frost-sensitive crops.
When is the first fall frost in Multnomah County, OR?
The median first fall frost in Multnomah County arrives around October 27. In cold years it can arrive as early as October 9; in mild years as late as November 19. Harvest or protect frost-sensitive crops — tomatoes, peppers, basil, squash — before this date.
How long is the growing season in Multnomah County?
Multnomah County has a frost-free growing season of approximately 196 days. This is enough time for most warm-season crops including tomatoes, peppers, and squash with proper timing. Climate records show the growing season is trending longer by about 1.6 days per decade.
What is the soil like in Multnomah County for gardening?
Multnomah County has predominantly Silt Loam soil with a pH range of 5.8–6.3 and Excessively Drained drainage. Most vegetables and herbs grow well with standard composting and seasonal soil amendment.
What is grown commercially in Multnomah County?
Multnomah County has commercial agriculture that includes Wheat, Hay, Apples, Potatoes, Cattle. These crops reflect the local climate and soil conditions — what succeeds commercially often translates well to home gardens in the same area.
Is Multnomah County a good location for home gardening?
Multnomah County scores 67/100 (Good) on our Microclimate Index, which combines frost reliability, drought pressure, soil challenge, elevation risk, and long-term climate trend. Conditions here are moderate — most common crops grow well with standard timing and care.
Your Multnomah County Garden Planner — Free
A 22-page printable planner built for Multnomah County (Zone 9a). 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