When To Plant Tomatoes In Seattle

If you’re planning your Seattle garden, knowing when to plant tomatoes is your first step to a great harvest. The best time for planting tomatoes here requires careful timing to beat our cool, wet springs and make the most of our sunny summers.

Our unique Pacific Northwest climate means you can’t just follow generic advice. Planting too early risks cold soil stunting your plants, while planting too late means your fruit might not ripen before the fall rains return. This guide will walk you through the specific dates, signs to watch for, and techniques that work best in the Puget Sound region.

For most Seattle area gardeners, the safe window to transplant tomato seedlings outdoors falls between mid-May and early June. The classic Memorial Day weekend is a popular target. This timing is all about soil temperature, not just the last frost date.

Our air might feel warm in April, but the soil stays cold for weeks longer. Tomatoes need soil consistently above 55°F to grow well. Planting into cold soil can shock plants, causing them to sit dormant and become vulnerable to disease.

Key Dates and Temperature Cues

While calendar dates are a helpful guide, your garden’s microclimate and the current year’s weather are the real deciders. Here’s a breakdown of what to consider:

Absolute Earliest (Risk-Takers): Some gardeners use protection like Wall O’ Waters or cold frames to plant out in late April. This is a gamble and requires daily attention to weather forcasts.
Early to Mid-May: Possible if you have a very warm, south-facing slope and plan to use cloches or row covers at night. Monitor soil temps closely.
Mid-May to Early June (Recommended): The safest bet for most. The soil has warmed, and night temperatures are more reliably above 50°F.
After June 15th: Planting too late can be a problem for long-season heirloom varieties, as they may not ripen fully before autumn coolness sets in.

How to Check If Your Soil Is Ready

Your best tool is a simple soil thermometer. Take the temperature at a depth of 2-3 inches, around 10-11 AM, for several consecutive days. If it’s holding at 55°F or higher, you’re getting close. Also, watch nature’s signals: when dandelions are in full bloom and the risk of a hard frost has passed, conditions are improving.

Starting Seeds Indoors vs. Buying Starts

Because of our long, cool springs, tomatoes must be started indoors well before outdoor planting time.

Starting Seeds Indoors: Begin seeds 6-8 weeks before your planned transplant date. For a mid-May transplant, start seeds in late March. You’ll need a strong light source, like a grow light, to prevent leggy seedlings.
Buying Nursery Starts: This is a great option for beginners. Look for stocky, dark-green plants without flowers or fruit already set. The best time to buy is often early May, but hold them in a sheltered spot until your garden soil is truly ready.

Choosing the Right Tomato Varieties for Seattle

Variety selection is crucial for success. Focus on types with shorter “days to maturity” (under 75 days) and those known for cooler climate performance.

Early Season & Cool Climate Stars: ‘Oregon Spring’, ‘Early Girl’, ‘Stupice’, ‘Glacier’. These set fruit in cooler temps.
Cherry & Salad Tomatoes: ‘Sungold’, ‘Sweet Million’, ‘Black Cherry’. They ripen quickly and prolifically.
Determinate (Bush) Types: Like ‘Celebrity’ or ‘Patio’. They produce a big, concentrated harvest, good for beating early blight.
Heirlooms for the Patient: ‘Brandywine’ and other long-season types can be grown but need the warmest, sunniest spot and often a season-extending tactic.

Step-by-Step Planting Guide for Seattle Gardens

Harden Off Your Plants: For 7-10 days before planting, gradually expose seedlings to outdoor conditions. Start with a few hours of dappled shade and sheltered breeze, increasing time and sun exposure each day. This prevents sunscald and shock.
Prepare the Planting Hole: Dig a hole much deeper than the root ball. Tomatoes can develop roots all along their stems, so we plant them deep. Mix a handful of balanced organic fertilizer or a specific tomato food into the bottom of the hole.
Plant Deeply: Remove the lower leaves and bury the stem up to the first set of true leaves. If the plant is leggy, you can even trench it—laying the stem sideways in a shallow trench and bending the top gently upward. This creates a massive, robust root system.
Water and Mulch: Water the plant in thoroughly to settle the soil. Then, apply 2-3 inches of mulch (like straw or compost) around the base, keeping it a few inches from the stem. This conserves moisture, suppresses weeds, and keeps soil temperature more even.
Provide Support Immediately: Install your cage, stake, or trellis right away to avoid damaging roots later. Our wet springs can cause rapid growth, and you want the plant supported from the start.

Protecting Your Tomatoes from Seattle’s Weather

Even after the safe planting date, our weather can suprise us. Here’s how to protect your investment:

Frost Protection: Keep row covers or old sheets handy for unexpected late cold snaps. Cover plants in the evening if temps are predicted to dip near 40°F.
Wet Soil & Disease: Our biggest challange is often fungal diseases like early blight, exacerbated by wet foliage. Water at the base of the plant, not overhead. Ensure good air circulation by proper spacing and pruning.
Cool Summer? In a particularly cool, cloudy summer, fruit set can be poor. Blossom set spray can sometimes help, but choosing the right varieties from the start is the best defense.

Season Extension Tips for Fall

To get the last tomatoes to ripen, use season extension techniques in the fall as well.

In late summer, pinch off new flower clusters so the plant’s energy goes into ripening existing fruit.
If frost threatens while you still have green fruit, you can pull the entire plant and hang it upside down in a garage or basement; many fruit will continue to ripen.
Green tomatoes can also be harvested and ripened indoors on a counter, though flavor is best when vine-ripened.

Common Mistakes to Avoid

Planting by the Calendar Alone: Ignoring soil temperature is the number one mistake.
Skipping Hardening Off: This step is non-negotiable for healthy transplants.
Overcrowding: Give plants at least 24-36 inches between them for air flow. Good circulation is your best defense against disease in our humid climate.
Inconsistent Watering: This leads to problems like blossom end rot. Use mulch and water deeply and regularly once summer heat arrives.

FAQ: Planting Tomatoes in the Pacific Northwest

Q: What is the last frost date for Seattle?
A: The average last spring frost date for the Seattle area is around March 28th, but frost can occure into April in outlying areas. More importantly, the soil remains cold long after the last frost.

Q: Can I plant tomatoes in July in Seattle?
A> It’s not advisable. The plants need enough warm weeks to grow, flower, and set fruit before temperatures drop in September. July planting would only work for very fast-maturing cherry types, and even then the yeild will be small.

Q: Should I use black plastic to warm the soil?
A: Yes, this is a very effective technique. Laying black plastic over your planting bed 2-3 weeks before your target date can raise soil temperature by several degrees. Just cut an X to plant through it.

Q: What about growing tomatoes in containers?
A: Containers are an excellent choice! They warm up faster than in-ground soil. You can plant a bit earlier if the container is in a sheltered, sunny location. Just ensure the pot is large enough (at least 15-20 gallons for full-sized plants) and has excellent drainage.

Q: Why did my plants grow tall but not set fruit?
A> This is often due to cool night temperatures during flowering. Tomatoes need nights above 55°F for good pollen production and fruit set. Our early summer sometimes fails to provide this. Choosing cooler-set varieties, as mentioned above, is the best solution.

Timing your tomato planting correctly in Seattle sets the stage for the whole season. By waiting for warm soil, choosing resilient varieties, and being prepared to protect your plants, you’ll greatly increase your chances of a bountiful, sun-ripened harvest. Pay close attention to your garden’s specific conditions each year, and you’ll develop a feel for the perfect planting moment.