An in-depth guide to starting seeds indoors, understanding germination, cold stratification, and growing healthy transplants in the Pacific Northwest.
By David Thompson
Founder/Director, Urban Harvest Exchange
WSU Pierce County Master Gardener
Every successful garden begins with a seed.
There's something remarkable about placing a tiny seed into soil and watching it develop into a plant that can eventually produce pounds of food, beautiful flowers, or herbs for the kitchen.
Starting your own seeds is one of the most rewarding and economical skills a gardener can learn. It allows you to grow varieties that aren't commonly available at garden centers, reduce gardening expenses, and produce healthy plants suited to your growing conditions.
Here in the Pacific Northwest, where cool springs and unpredictable weather can make gardening challenging, starting seeds indoors gives us a valuable head start.
At Urban Harvest Exchange, we start vegetable, herb, and flower seeds in our greenhouses for our community gardens and free plant-start giveaways.
With Tacoma Urban Farm expanding to 42 raised beds in spring 2027, seed starting will be an important part of producing more food for our community.
But successful seed starting requires more than putting seeds in soil and adding water.
Understanding how seeds germinate, what conditions they need, and why some seeds require special treatment can dramatically improve your success.
Let's start at the beginning.
Understanding How Seeds Germinate
A seed is a living plant embryo surrounded by stored nutrients and a protective seed coat.
Under the right conditions, the embryo begins growing and eventually develops into a seedling.
Most seeds need three basic things to germinate:
1. Moisture
Water activates metabolic processes inside the seed, allowing the embryo to begin growing.
The growing medium should remain evenly moist but never waterlogged.
Too little moisture prevents germination. Too much can deprive the seed of oxygen and encourage rot.
2. Oxygen
Seeds need oxygen for respiration.
A seed-starting mix that remains saturated can prevent oxygen from reaching the seed.
This is why good drainage and a light, well-aerated growing medium are essential.
3. Appropriate Temperature
Different seeds germinate best at different temperatures.
Warm-season vegetables such as peppers and tomatoes generally require warmer conditions than lettuce, peas, or many brassicas.
Some seeds also require specific light conditions, scarification, or a period of cold, moist treatment before germinating.
These special requirements are adaptations that help seeds germinate at the appropriate time in nature.
Why Start Seeds Indoors?
Starting seeds indoors offers several advantages, especially in Western Washington.
A Longer Growing Season
Our cool spring weather can delay the planting of heat-loving vegetables.
By starting tomatoes, peppers, and other crops indoors, we can have established plants ready to transplant when outdoor conditions become suitable.
More Variety
Garden centers offer a limited selection of plants.
Starting from seed opens the door to hundreds of varieties, including heirlooms, open-pollinated vegetables, unusual herbs, and flowers that support pollinators.
Lower Costs
A packet of seeds can produce many plants for considerably less than purchasing individual transplants.
This is especially important for community gardens and organizations like Urban Harvest Exchange, where we're growing plants for both our gardens and free community distribution.
Better Control Over Plant Health
Starting your own plants allows you to control the growing medium, watering, light, and fertilizer.
You can also monitor plants from germination through transplanting.
Food Independence
Learning to start seeds gives gardeners another practical skill for producing food.
Combined with seed saving, composting, and sustainable gardening practices, seed starting helps build greater local food sovereignty.
When to Start Seeds in the Pacific Northwest
One of the most common mistakes gardeners make is starting seeds too early.
It's tempting to begin planting tomatoes and peppers in January, especially after months of cold, rainy weather.
But seedlings started too early may become tall, weak, rootbound, or difficult to maintain before they're ready to go outdoors.
The goal isn't to grow the largest transplant.
The goal is to grow a healthy, compact plant that's ready to transplant at the right time.
In the Tacoma area, our average last spring frost is generally sometime in spring, but the exact date varies by neighborhood, elevation, and microclimate.
More importantly, frost-free weather doesn't necessarily mean the soil is warm enough for tomatoes, peppers, or squash.
Use local weather conditions, soil temperatures, and the requirements of each crop rather than relying on one calendar date.
General Indoor Seed-Starting Schedule for Tacoma
CropStart indoorsTransplant outdoorsOnions and leeksJanuary–FebruaryMarch–April, when conditions permitBroccoliFebruary–MarchMarch–AprilCabbageFebruary–MarchMarch–AprilCauliflowerFebruary–MarchMarch–AprilLettuceFebruary–AprilMarch–MayKaleFebruary–AprilMarch–MayTomatoesMarch–AprilMay–June, after soil warmsPeppersFebruary–MarchLate May–June, after soil warmsEggplantFebruary–MarchLate May–JuneBasilMarch–AprilLate May–JuneCucumbersApril–MayMay–JuneSquash and zucchiniApril–MayMay–June
These are general guidelines. Seed packet instructions and local conditions should always take priority.
Peppers and eggplant often need a longer indoor growing period than tomatoes.
Cucumbers and squash grow quickly and generally shouldn't be started too far ahead of transplanting.
Many crops, including carrots, radishes, beans, and peas, are usually better direct-sown into garden soil rather than transplanted.
Choosing the Right Seed-Starting Supplies
You don't need expensive equipment to start seeds successfully.
However, a few basic supplies can make a substantial difference.
Containers
You can use:
Commercial seed-starting trays.
Cell packs.
Reusable plastic pots.
Soil blocks.
Clean, repurposed containers with drainage holes.
Whatever you choose, drainage is essential.
If reusing containers, wash them thoroughly and sanitize them when appropriate to reduce disease risks.
Seed-Starting Mix
A quality seed-starting mix should be lightweight, well-drained, and capable of holding moisture without becoming waterlogged.
Commercial seed-starting mixes commonly contain ingredients such as peat moss, coconut coir, perlite, or vermiculite.
Avoid using ordinary garden soil in small indoor containers.
Garden soil can compact, drain poorly, and introduce disease organisms.
Some seed-starting mixes contain little or no fertilizer. Once seedlings develop true leaves, they may need supplemental nutrients.
Grow Lights
Light is one of the most important factors in growing healthy seedlings.
A sunny windowsill often doesn't provide enough light during our cloudy Pacific Northwest winters.
Insufficient light causes seedlings to become tall, pale, and weak, a condition commonly called legginess.
LED grow lights are an energy-efficient option.
For many seedlings, approximately 14–16 hours of light daily is a useful starting point, followed by a period of darkness.
Adjust light height and intensity according to the fixture manufacturer's instructions and the needs of the plants.
Heat Mats
Heat mats warm the growing medium and can improve germination for heat-loving crops.
They are particularly useful for:
Peppers.
Tomatoes.
Eggplant.
Basil.
Some flowers and herbs.
A thermostat-controlled heat mat is preferable because it helps prevent overheating.
Once seeds germinate, many seedlings grow best at temperatures somewhat cooler than their optimum germination temperature.
Labels
Always label your containers when planting.
Include the crop, variety, and sowing date.
It's surprisingly easy to forget which tomato or pepper variety you planted when all the seedlings look alike.
Step-by-Step Instructions for Starting Seeds Indoors
Step 1: Prepare Your Growing Medium
Moisten the seed-starting mix before filling your containers.
The mix should feel like a wrung-out sponge: damp but not dripping wet.
Pre-moistening helps prevent dry pockets and reduces the need for heavy watering after planting.
Step 2: Fill Your Containers
Fill seed trays or pots loosely with the moistened mix.
Gently settle the medium without compacting it excessively.
Leave a little room at the top for watering.
Step 3: Plant at the Correct Depth
A common rule is to plant seeds at a depth approximately two to three times their diameter.
However, this is only a general guideline.
Some very small seeds need light to germinate and should be pressed onto the surface rather than buried.
Always follow the instructions for the specific species.
Step 4: Water Carefully
After sowing, water gently so seeds aren't displaced.
Bottom watering is an excellent technique.
Place the containers in a shallow tray of water and allow the growing medium to absorb moisture from below.
Remove excess water after the medium is adequately moistened.
Don't leave seed trays standing in water for extended periods.
Step 5: Provide the Right Temperature
Place the trays where the growing medium will remain within the crop's preferred germination range.
Use a heat mat when appropriate.
Not every crop benefits from additional warmth. Lettuce and some cool-season plants may germinate poorly when temperatures become too high.
Step 6: Monitor Germination
Check your trays daily.
Keep the growing medium evenly moist, but don't saturate it.
If using humidity domes, remove or ventilate them promptly as seedlings emerge to reduce excessive humidity.
Step 7: Provide Strong Light
As soon as seedlings emerge, provide adequate light.
Don't wait several days before moving them under grow lights.
Weak lighting during the first days of growth can produce stretched seedlings that never develop as strongly as properly grown plants.
Step 8: Begin Fertilizing When Needed
Once seedlings develop their first true leaves, they may need additional nutrients, depending on the growing medium.
Use a suitable diluted fertilizer according to the product instructions.
Avoid overfertilizing young seedlings, which can damage tender roots.
Step 9: Pot Up When Necessary
If seedlings outgrow their cells before outdoor planting conditions are suitable, transplant them into larger containers.
Handle seedlings by their leaves rather than squeezing delicate stems.
Most seedlings should be transplanted at approximately their original depth. Tomatoes are an exception because they can develop additional roots along buried stems.
Understanding Seed Dormancy
Not all seeds germinate immediately when given water and warmth.
Many species have evolved mechanisms that prevent germination until environmental conditions indicate a favorable growing season.
This is called seed dormancy.
Dormancy helps protect plants from germinating at the wrong time.
For example, some seeds naturally fall to the ground in autumn but shouldn't germinate immediately before winter.
Instead, they remain dormant until they've experienced a period of cold, moist conditions.
Other seeds have hard seed coats that prevent water from entering until the coat is weakened.
Two important techniques used to overcome dormancy are:
Stratification: Exposing seeds to specific temperature and moisture conditions to overcome physiological dormancy.
Scarification: Weakening or opening a hard seed coat so water can enter.
Understanding the difference can greatly improve germination success.
Cold Stratification: Why Some Seeds Need Winter Before Spring
Cold stratification is a technique that imitates the natural winter conditions certain seeds need before they will germinate.
In nature, seeds fall to the ground during late summer or autumn.
They experience weeks or months of cool, moist conditions.
During that period, physiological changes occur inside the seed that help overcome dormancy.
When temperatures rise in spring, the seeds become capable of germinating.
Gardeners can reproduce these conditions artificially.
Cold stratification is particularly important for many native plants, perennial flowers, trees, shrubs, and certain herbs.
Most commonly grown annual vegetables, including tomatoes, peppers, beans, peas, lettuce, and squash, do not require cold stratification.
Plants That May Require Cold Stratification
Examples include certain species or varieties of:
Milkweed.
Echinacea.
Lavender.
Delphinium.
Columbine.
Black-eyed Susan.
Native perennial wildflowers.
Various trees and shrubs.
Requirements vary by species and even by seed source.
For example, some commercially produced echinacea and lavender seeds germinate without stratification, while others benefit from it.
Always check species-specific instructions before beginning treatment.
How to Cold Stratify Seeds in Your Refrigerator
For many gardeners, refrigerator stratification is the simplest and most reliable method.
Materials Needed
Seeds requiring cold stratification.
A clean plastic bag or small container.
Damp paper towel, vermiculite, or seed-starting medium.
Permanent marker and label.
Refrigerator.
Step 1: Research the Species
Determine whether the seed requires cold stratification and how long the treatment should last.
Many species benefit from approximately 30–60 days, but some require shorter or much longer periods.
Some species require warm stratification followed by cold stratification, so don't assume one method works for everything.
Step 2: Prepare the Moist Medium
Moisten a paper towel, vermiculite, or other clean medium.
It should be damp but not dripping wet.
Too much moisture can cause mold or seed rot.
Step 3: Combine Seeds and Medium
Place the seeds in contact with the moist medium.
For small seeds, gently distribute them across the surface.
Step 4: Place in a Labeled Container
Put the seeds and medium into a plastic bag or suitable container.
Label it with:
Plant species and variety.
Date stratification began.
Intended treatment duration.
Expected completion date.
Step 5: Refrigerate
Place the container in the refrigerator at approximately 34–41°F (1–5°C), unless the species requires a different temperature.
Do not freeze seeds that specifically require moist chilling above freezing.
Step 6: Check Regularly
Inspect the seeds every week or two.
Make sure the medium remains slightly moist.
Watch for mold, decay, or early germination.
If seeds begin germinating during stratification, plant them promptly and carefully.
Step 7: Plant After Stratification
When the required chilling period is complete, sow the seeds according to their specific germination requirements.
Some species need light, while others need darkness.
Provide the appropriate temperature and moisture conditions.
Important: Stratification doesn't guarantee germination. Seed age, viability, dormancy type, and species-specific requirements still matter.
Outdoor Cold Stratification: Let Nature Do the Work
Here in Western Washington, we can sometimes use our naturally cool, moist winters to stratify seeds outdoors.
This method is often called winter sowing or outdoor stratification, although winter sowing can also be used for species that don't require stratification.
How to Do It
Step 1: Select a species suited to outdoor winter sowing.
Step 2: Fill a container with a well-draining growing medium.
Step 3: Sow the seeds at the recommended depth.
Step 4: Protect the container from animals, excessive rain, and seed loss while allowing natural temperature changes.
Step 5: Place it outdoors in a sheltered location.
Step 6: Monitor moisture throughout winter.
Step 7: Watch for germination as temperatures become favorable.
Some gardeners use recycled translucent milk jugs as miniature greenhouses.
If using this technique, provide drainage holes and ventilation, and avoid allowing the container to overheat on sunny days.
One advantage of outdoor stratification is that seeds experience natural seasonal conditions.
However, winter temperatures in the Tacoma area may not provide the exact chilling requirements for every species.
Scarification: Helping Hard-Coated Seeds Germinate
Some seeds have physical dormancy caused by hard, water-resistant seed coats.
In nature, weathering, soil microorganisms, temperature changes, or other environmental processes may gradually weaken those coats.
Scarification helps overcome this barrier.
Mechanical Scarification
For suitable large, hard-coated seeds:
Identify the seed coat and avoid damaging the embryo.
Gently rub a small area with fine sandpaper or a nail file.
Stop when the outer coat is slightly thinned.
Soak the seed if recommended for that species.
Sow promptly.
Not all seeds should be scarified. Doing so unnecessarily can damage otherwise viable seeds.
Soaking Seeds
Some seeds benefit from soaking in room-temperature water before planting.
The soaking period depends on the crop, but many commonly soaked seeds need no more than several hours to overnight.
Avoid prolonged soaking, which can deprive seeds of oxygen and encourage decay.
What About Hot Water?
Certain hard-coated seeds have specialized hot-water scarification requirements.
Because temperatures and exposure times are species-specific, follow reliable instructions rather than experimenting with boiling water.
Excessive heat can kill the embryo.
Common Seed-Starting Problems and How to Prevent Them
Damping-Off Disease
Damping-off is a group of diseases that can cause seedlings to collapse near the soil line.
It is often associated with excessive moisture, poor airflow, contaminated equipment, and unfavorable growing conditions.
To reduce the risk:
Use clean containers.
Use fresh, well-draining growing medium.
Avoid overwatering.
Provide adequate airflow.
Remove humidity domes after emergence.
Avoid overcrowding.
Leggy Seedlings
Tall, thin, weak seedlings usually indicate inadequate light, although excessively warm conditions can contribute.
Provide sufficient light intensity and appropriate growing temperatures.
Seeds That Won't Germinate
Possible causes include:
Old or nonviable seed.
Incorrect temperature.
Too much or too little moisture.
Planting too deeply.
Incorrect light conditions.
Dormancy requirements that haven't been met.
Before discarding a tray, check the normal germination period for that species.
Some seeds germinate in just a few days, while others take several weeks or longer.
Yellowing Seedlings
Yellow leaves can indicate nutrient deficiencies, waterlogged roots, insufficient light, or other growing problems.
Identify the likely cause before adding fertilizer.
Hardening Off: Preparing Seedlings for Outdoor Life
One of the biggest mistakes gardeners make is moving seedlings directly from indoor growing conditions into full outdoor exposure.
Indoor-grown seedlings aren't accustomed to wind, direct sunlight, or fluctuating temperatures.
Hardening off is the gradual process of acclimating seedlings to outdoor conditions.
A Simple 7–10 Day Hardening-Off Schedule
Days 1–2: Place seedlings outdoors in a sheltered, shaded location for a few hours.
Days 3–4: Gradually increase their time outside and introduce gentle morning sunlight.
Days 5–6: Increase sunlight and exposure to normal outdoor conditions as appropriate.
Days 7–10: Allow longer outdoor periods while protecting plants from damaging cold, strong winds, and intense sunlight.
Move plants indoors or provide protection whenever temperatures fall below their tolerance.
Warm-season vegetables such as peppers, tomatoes, basil, and eggplant require particular care.
Hardening off doesn't make frost-sensitive plants frost-proof.
Starting Seeds for Fall and Winter Gardens
Seed starting isn't limited to spring.
In the Pacific Northwest, many vegetables intended for fall and winter harvest should be started during summer.
For example:
Kale and collards can be started in summer for fall and winter production.
Broccoli and cabbage can be started for fall harvest.
Lettuce and spinach can be started in late summer.
Certain Asian greens can be grown for autumn harvest.
Starting fall crops indoors or in a protected area can help seedlings survive hot, dry summer conditions before transplanting.
Remember that winter vegetables need time to become established before daylight drops below approximately ten hours per day.
Good timing is essential.
Seed Starting at Urban Harvest Exchange
At Tacoma Urban Farm, seed starting is one of the foundations of our community gardening programs.
We use our greenhouses to grow vegetable, herb, and flower starts for our growing locations and free community giveaways.
Our goal is to make growing food accessible to people who may not have the equipment, space, or experience to start seeds themselves.
Instead of simply handing someone a packet of seeds, we can provide a healthy plant ready to grow.
That plant might produce tomatoes, peppers, herbs, or other vegetables throughout the season.
For someone on a limited income, even a few productive plants can help supplement their groceries.
Through our community gardens, Food Independence Gardens, and plant-start giveaways, we work to turn donated seeds into fresh food and greater food independence.
With 42 raised beds planned at Tacoma Urban Farm for spring 2027, we'll need more seeds, growing supplies, and volunteer support than ever.
Seed donations from companies and individuals help us make this work possible.
Ten Tips for Successful Seed Starting
Start seeds at the right time rather than as early as possible.
Use clean containers with drainage holes.
Choose a lightweight, well-draining seed-starting mix.
Follow species-specific planting depth instructions.
Maintain appropriate germination temperatures.
Provide strong light immediately after seedlings emerge.
Keep growing media moist, not saturated.
Label every variety and record sowing dates.
Research whether unusual seeds require stratification or scarification.
Harden off seedlings before transplanting them outdoors.
Final Thoughts: Every Seed Is an Opportunity
A seed may be small, but its potential is enormous.
One tomato seed can produce a plant that yields pounds of food.
One bean seed can produce dozens of new seeds.
One flower seed can grow into a plant that supports pollinators and contributes to a healthier garden.
And one person who learns to start seeds successfully can share that knowledge with others for years to come.
Seed starting is more than a gardening activity.
It's an investment in food security, environmental stewardship, and community resilience.
Whether you're growing vegetables on a balcony, starting flowers for pollinators, or producing hundreds of plants for a community garden, the principles are the same.
Understand what your seeds need, provide the right conditions, and give them time to grow.
The future of our food system begins with the seeds we plant today.
Rescue the Harvest – Nourish the People.
David Thompson
Founder/Director, Urban Harvest Exchange
WSU Pierce County Master Gardener
