Skagit County Master Gardener Foundation

Cultivating plants, people and communities since 1977


Growing Tomatoes from seed in the Pacific Northwest

Growing your own tomato starts is not a secret, just sound gardening practices and 10 weeks of careful nurturing in order to transplant young plants outdoors in May/June.

What kind of tomato do you want to grow?

What Variety to Plant

There are many varieties of tomatoes to suit almost every taste and growing condition.  When selecting a variety, look at the following:

Days to Harvest/Days to Maturity
Days to harvest is roughly how long before you can expect ripe fruit.  Days to maturity can also be a guide to distinguish varieties as early (65 days or less), mid-season (65 to 80 days) and late-season (80 days or more).   For the Pacific Northwest, gardeners should mostly look at early varieties unless they have a greenhouse.

Growing Days
All days are not created equal for growing plants.   Your goal in starting plants indoors is to maximize the available growing season.  Growing Degree Days (GDD) are days that met or exceed a base temperature where the plants receives adequate warmth for tissue growth.  Base temperatures vary by crop type.  Wheat requires less warmth to grow than a tomato.  The base temperature for warm season crops such as tomatoes and peppers is 50 °F.  GDD is calculated by subtracting the base temperature (50 °F in this case) from the average daily temperature (add the high and the low temperature for the day and divide by two).  If the resulting value is less than 0, then it is set to 0.  GDD units are always positive.  If it is above 50, it is a GDD.  If a tomato variety lists 60 days to harvest, you will need 60 GDD for that plant to produce ripe fruit.  Regional GDD data is maintained for growers to estimate when to plant based on historical averages.  Click here for the WSU Ag Weather GDD data for Mount Vernon. 

Determinate or Indeterminate
Determinate varieties of tomatoes produce many short branches with flowers and fruit on the ends.  They are usually early varieties and the fruit generally ripens at the same time.  These smaller plants are ideal for patio pots and small greenhouses.  Indeterminate varieties continue to grow and produce flowers and fruit all season.  The vines can grow over 25 feet in optimal conditions and will need support from stakes or cages.  Semi-determinate plants are more compact than indeterminate, but keep producing until frost.

Disease Resistance
Resistance to diseases is important to consider.  Many variety names are followed by one or more letters.  For example, Verticillium wilt is "V" and Fusarium wilt is "F".

How to choose? 
Tomato fruit can range in size from small cherry tomatoes to fruits to as large as grapefruit.   Many colors are available.  How you will use the tomatoes?  Are you looking for slicing tomatoes or salad tomatoes?  There are low moisture, low seed varieties suited for canning or drying.  Heirloom or heritage tomatoes are traditional non-hybridized varieties. Use the Variety Selection Tool above, and check out varieties that the Master Gardeners have tested in the Skagit Valley area.


Tomato seedlings need 14 to 16 hours of light per day to be successful; unfortunately, natural light through a window will not provide this in late winter/early spring. Florescent or LED tube lights are a great, inexpensive solution. Tube lights can easily be positioned close to the top of the seedlings.

You do not have to use a “plant light.” Plant lights offer full spectrum and high intensity lighting which is great if you are looking to have a plant bloom and fruit without adequate sunlight. Since we are just starting plants indoors, we can cheat a bit on their lighting needs and save a lot of money.

Be creative to keep your lights positioned 2-3 inches above growing plants.

DIY Lighting Tips

  • Hang a shop light from a book shelf.
  • Bungee cord a shop light to the bottom of a table.
  • Rest a board on and between two chairs. Hang a shop light on “I” hooks below the board. Place plants on the floor under the light.
Starting Seeds Indoors to Extend Your Harvest

How Many to Plant

Think about how may tomatoes you need and your garden space.  Most gardeners tend to over plant.  It is better to plant fewer tomatoes and pay closer attention to them.  Less tomato plants per square foot allow more air circulation and sunlight to the leaves.  Once you have determined how many plants you want, plant 20% more seeds than the number you desire.  Tomatoes usually germinate easily.  Remember you can thin them or give away extras.

When to Plant
Due to the short growing season in Western Washington, Master Gardeners recommend that you plant seeds indoors on heat mats in March.  This will result in plants 8 weeks old by the last frost in May.  Some areas of Skagit Valley vary slightly due to micro-climates, so experiment to see what works for you.  If you want to know your frost dates, got to the WSU AgWeatherNet.

Prepare Starting Media
Moisten fertilizer free seed starting mix with warm water to make it usable.  Make sure to add a little water at a time and mix thoroughly.

Germination Container
If your tomato seeds get sufficient moisture and warmth, almost any container will work for germination. Plant protector trays, butter tubs, clam shell food containers, etc.  Previously used containers should be sterilized with a light bleach solution (5-10%).

Planting the Seeds
Fill the container with 3/4” of the pre-moistened seed starter mix.  Sprinkle the seeds on top.  Then dust the surface with dry starter mix to cover the seeds.  Gently water with spray bottle.  Tightly close the lid or place in a plastic bag to retain moisture.  Label the container with the variety of tomato seed and date planted.  Do not let the seedlings dry out.  Keep the soil moist to very moist. No lights are needed at this stage.  It is ok for them to be in the dark.

Your tomato seeds will germinate in 4 to 10 days.  Check daily.  When the first seeds sprout, remove the covering and return the container to the heat mat.  Add artificial light 14-16 hours/day.  Keep lights continually adjusted 2-3 inches above the plants or as per the instructions for grow lights.  Water with a spray bottle to keep soil moist to very moist.  Soil can dry out quickly on a heat mat.  Do not let the seedlings dry out.   A small fan provides good ventilation and the motion helps strengthen the stems.

Potting Up

Transplanting into 6 Packs
Seedlings are ready to transplant to 6 pack plug trays any time after the first cotyledon leaves have unfolded up until the first true leaves fully emerge.  This will be as early as 5 days or up to 3 weeks.  It is easier to untangle the roots of smaller plants, and you want to give the roots room to grow as soon as possible.

Fill 6 pack containers with seed starter mix.  Poke a hole in the center of each plug using a chop stick or pencil.  Carefully pull seedlings from the dish by their small leaves untangling the roots, if necessary, with a plastic fork or your fingers.  DO NOT pull the seedling out by the stem as it is delicate and can cause disease if bruised or otherwise compromised.  Remember to label your plants each time you transplant them.

Plugs need light, heat and a low nitrogen fertilizer.  Water with water soluble 12-31-14 fertilizer (1 tsp/gal) at each watering.  Keep the soil moist but not wet.  Tomatoes love warm water!  Be creative to keep your seedlings 2-3 inches under low intensity lights for 14-16 hours/day. If the light is not adequate, you will get long, leggy stems shooting up.

Transplant to 4" Pots
Transplant when the roots are visible at the bottom or sides of the plug. This will take about 7 days to 2 weeks.   Fill your 4 inch pots with a mixture of 5 parts potting soil and 3 parts perlite.  To remove the plants from the 6 pack containers, gently squeeze the bottom of each cell of the container or scoop out the contents with a teaspoon.  If that doesn’t work, take a pair of scissors and cut lengthwise down each cell.  This will make it easy to remove the plug.

Prepare the 4" pot by making a hole with a dibble or other object (chop stick or pencil) and drop in the plug.  Plant the seedlings to the lowest leaves.  You can pinch off the cotyledon leaves to bury the plants deeper.  Water gently with a spray or squeeze bottle.  Do not let plugs dry out or over water.  Slightly moist is perfect.

Transplanting to Gallon Pots
When are the tomato plants ready for gallon pots?  If you are past your average last frost date, you may be able to skip this step and plant outside.  Otherwise, it’s time to transfer your plants from 4" pots to gallon pots when the roots reach the sides or bottom of the 4" pot.  Continue to use a mixture of 5 parts potting soil and 3 parts perlite. Rough up the root ball and bury as much of the plant stem as is reasonable. The stem, when buried, will develop roots and make for a more vigorous plant. Water with a water soluble 17-17-17 fertilizer (1 tsp/gal) at each watering.

Caring for Tomato Starts Until the Last Frost

Unheated Greenhouse
If weather permits, your plants will thrive in an unheated greenhouse, 40-75 degrees F. At this stage, they no longer need a heat mat.  However, if the weather remains cloudy, you may need to continue artificial lighting.  If the temperature drops, you can use a small heater and/or cover the trays with a plastic dome lid at night.  Water carefully and leave a fan on.  Cool plants grow thick stems and robust roots.

Basement or Garage
Be creative to duplicate greenhouse requirements.  Find a spot where the temperature is 40-75 degrees.  Continue artificial lighting for 14-16 hours/day.  Use a heat mat if location is under 50 degrees at any time.

Hardening Off
Tomatoes that have been grown in protected environments (this includes most store bought plants) need to be properly prepared before planting outdoors.  Two weeks before transplanting, start setting plants outdoors for a few hours each day to acclimate them gradually to outdoor conditions.  Protect them from rain and wind.  Leave them out a little longer each day.  Keep an eye on the soil moisture.  Be careful how long you leave them out on sunny days as they can get sun burn.

Continue to bring plants in at night until night time temperatures are above 50°F.   Chilled plants will be more likely to get catfacing or other physiological problems.

Be creative, give the tomatoes as much sun as possible.

Keep them dry and warm at night.

Improvise your own tomato cart.

Finishing Tomatoes Outside
After the Last Frost

Read more about planting, pruning, and diseases from the WSU Extension Fact Sheet.