Starting Seeds Indoors (2024)

Starting Seeds Indoors (1)

Photo: Erica Smith

Updated: February 20, 2023

Starting seeds indoors is about as much fun as a food gardener can have in late winter! The idea is to grow baby plants (a.k.a. transplants or starts) for 2-8 weeks (depending on the vegetable and rate of plant growth) and then plant outdoors where the crops will mature and be harvested. Just about any crop can be started inside and transplanted outside. With a small investment and a bit of space, you can grow hundreds of healthy transplants. All of the supplies you need can be found at home (reused food containers for starting seeds) or purchased locally from hardware stores, garden centers, and big box stores.

Growing your own transplants from seeds

  • saves you money -- this may take a few years since there are first-year set-up costs
  • increases your garden’s output -- get earlier harvests by starting with transplants instead of seeds
  • allows you to grow the crops and cultivars you like best rather when you need them -- no need to plant only what’s available in retail stores
  • gives you better control of germination and plant stand -- fewer skips, no thinning
  • lessens pest and weather risks -- no worries about cool, wet weather keeping you from planting or encouraging seed rotting diseases.

When to plant vegetable seeds indoors

The proper time to sow seeds for transplants depends on when plants may safely be moved out-of-doors in your area. This period may range from 2-3 weeks (lettuce) to 8 weeks (pepper, eggplant) before transplanting, depending on the speed of germination and rate of growth (refer to“Germination Information for Selected Vegetable Crops” below).

A common mistake is to sow seeds too early and then attempt to hold the seedlings back under poor light or improper temperature ranges. This can result in tall, weak, spindly plants that do not perform well in the garden. Sow tomatoes 6-7 weeks before you expect to plant. You will end up with stocky 8-10 inchtall plants. If they do get too tall, you can lay them down in a trench when planting and turn the growing tip up so only the top 2-3 sets of leaves are above the soil.

Can I grow transplants on a sunny windowsill?

The natural light from a window is seldom enough for good, strong seedling growth. Plant stems usually stretch and lean towards the light and will not produce sturdy plants. Acceptable plant growth usually only occurs in south-facing bay windows or solariums. Light is measured by its intensity (closeness and brightness), duration (length of time the light is available), and quality (includes blue and red wavelengths). Fluorescent light fixtures and tubes are the best choices for growing transplants indoors.

What to plant indoors

  • Typical vegetable transplants found in garden centers in the spring include cabbage, broccoli, and cauliflower; followed by tomato, pepper, eggplant, squash, cucumber, melon, and lots of different herbs. You can grow all of these under your fluorescent lights PLUS the following: beets, spinach, Swiss chard, kale, mustard, broccoli raab, arugula, Asian greens, onion, leek, bean, and sweet corn!
  • Don’t forget about your need for mid-summer, late summer, and early fall transplants to keep your garden productive. It’s difficult to find vegetable transplants in stores beyond mid-June.
  • Use fresh seed or seed that has been stored properly from last year. Surplus seeds should be stored in a cool, dry location, like your freezer.

How to plant your seeds

You can grow two standard-sized (10.5 in. X 21 in.) trays under one 4 ft. long fluorescent fixture with two tubes. You can grow four trays under two fixtures. Each tray can hold about 12-18 large transplants and 50-120 small transplants.

Set up your lighting system

Starting Seeds Indoors (2)

  • Learn about options forgrow lights.

Choose containers and prepare growing medium

  • Choose yourcontainers and growing medium.
  • Place your dry growing medium into a bucket or tub. Pour lukewarm tap water in slowly and mix it into the growing medium. It should be as moist as a wrung-out sponge, not soppy wet. Water sprinkled on top of dry soilless media will bead up and fail to soak in.
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Fill containers

  • Fill your container to within ¾ of an inch from the top with the moistened medium.
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  • Firm the medium very lightly at the corners and edges with your fingers or a block of wood to provide a uniform, flat surface.

Mark your rows

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  • Good light and air movement results from sowing in rows, as compared to broadcasting the seed randomly across the surface. If "damping-off" disease appears, there is less chance it will spread. Seedlings grown in rows are easier to label and handle at transplanting time.
  • For small-to-medium size seeds (all crops except cucumber, squash, melons, corn, beans) make rows about 1- to 2-inches apart and 1/8”-1/4” of an inch deep across the surface of the container, using a narrow board or pot label.

Plant

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  • Sow the seeds thinly and uniformly in the rows by gently tapping the packet of seed as it is moved along the row. Lightly cover the seeds (lettuce seeds can be left on the surface un-covered or very lightly covered) and press down gently to ensure good contact between the seed and the soilless growing medium.
  • A suitable planting depth is usually about twice the diameter of the seed.
  • Sow large seeds (cucumber, squash, melons, corn, bean) directly into small containers or cell packs, eliminating the need for potting up latter. Sow two or three seeds per unit and later thin to allow the strongest seedling to grow.
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Starting Seeds Indoors (11)

Tips for quick germination

  • The seeds and growing medium need to be moist and warm to germinate. Generally, 65°-75°F is best for germinating seeds of most plants. This should be the temperature of the growing medium, not the air.
  • Seed germination begins with the absorption of water. An adequate and continuous supply is essential. Once the process has begun, a dry period will cause the death of the embryo. Spray some water from a plastic mister on the growing medium as needed, to keep it moist.
  • Cover the container with a piece of clear plastic or insert the container in a plastic bag. This will increase the humidity and temperature. The plastic should not be in contact with the growing medium. Remove the plastic as soon as sprouts appear.
  • You can buy heating pads to set your containers on to warm the growing medium and speed-up germination. A cheaper and easier approach is to drape clear plastic over your light fixture. The plastic should rest on the frame holding the fixture and not on the fixture itself. Leave the lights on and the heat from the ballast will be trapped inside the plastic tent and keep the temperature at 70º-75º F.
  • The top of your refrigerator is another good warm place for quicker germination.

Germination information for selected plants

PlantApproximate
time to seed
before last frost
date (weeks)
Time seeds
need to
germinate
(days)
Temperature
(F
Light/dark
requirement
Broccoli 8 5 to 10 70Either
Cabbage 8 5 to 10 70Either
Cauliflower 8 5 to 10 70Either
Cucumber 4 or less 5 to 10 85Either
Eggplant 8 5 to 10 70Either
Lettuce 8 5 to 10 70Either
Muskmelon 4 or less 5 to 10 85Either
Pepper 8 5 to 10 80Either
Squash 4 or less 5 to 10 85Either
Tomato 6 5 to 10 80Either
Watermelon 6 5 to 10 70Either

Related information

Grow Lights

Containers and Growing Media (Soil) for Indoor Seed Starting

Care of Vegetable Seedlings

Hardening Off

Poor Germination of Vegetable Seeds

Additional resources

Maryland GrowsBlog posts about seeds

Maryland GrowsBlog posts about growing transplants

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Starting Seeds Indoors

Starting seeds indoors is a popular practice among food gardeners in late winter. The goal is to grow baby plants, also known as transplants or starts, for a period of 2-8 weeks, depending on the vegetable and the rate of plant growth. These transplants are then planted outdoors, where they will mature and be harvested. Almost any crop can be started inside and later transplanted outside. This practice allows gardeners to have control over the germination and plant stand, reduces pest and weather risks, and provides the opportunity to grow preferred crops and cultivars.

Benefits of Starting Seeds Indoors

There are several benefits to starting seeds indoors. Firstly, it saves money in the long run, although there may be initial setup costs in the first year. Secondly, it increases the garden's output by allowing for earlier harvests compared to starting with seeds directly in the ground. Thirdly, it provides the flexibility to grow preferred crops and cultivars, rather than being limited to what is available in retail stores. Lastly, it allows for better control over germination and plant stand, resulting in fewer skips and no thinning.

When to Plant Vegetable Seeds Indoors

The proper time to sow seeds for transplants indoors depends on when it is safe to move the plants outdoors in your area. This period can range from 2-3 weeks before transplanting for lettuce to 8 weeks before transplanting for peppers and eggplants. The timing depends on the speed of germination and the rate of growth for each vegetable. It is important not to sow seeds too early, as this can result in weak and spindly plants. For example, tomatoes should be sown 6-7 weeks before the expected planting date to ensure stocky and healthy plants.

Growing Transplants Indoors

To grow transplants indoors, it is important to provide adequate light. Natural light from a window is usually not sufficient for strong seedling growth, as the plants tend to stretch and lean towards the light. South-facing bay windows or solariums with good light intensity, duration, and quality are better options. However, fluorescent light fixtures and tubes are considered the best choice for growing transplants indoors.

What to Plant Indoors

A wide variety of vegetable transplants can be grown indoors, including cabbage, broccoli, cauliflower, tomatoes, peppers, eggplants, squash, cucumbers, melons, and various herbs. Additionally, beets, spinach, Swiss chard, kale, mustard, broccoli raab, arugula, Asian greens, onions, leeks, beans, and sweet corn can also be grown under fluorescent lights. It is important to note that mid-summer, late summer, and early fall transplants are also needed to keep the garden productive, as it can be difficult to find vegetable transplants in stores beyond mid-June.

How to Plant Seeds Indoors

To plant seeds indoors, you will need containers and a growing medium. Containers can be trays or pots, and the growing medium should be moistened to the consistency of a wrung-out sponge. The seeds should be sown in rows, with small-to-medium-sized seeds placed about 1-2 inches apart and 1/8-1/4 inch deep. Large seeds, such as cucumber, squash, melons, corn, and beans, can be sown directly into small containers or cell packs. It is important to maintain the moisture and temperature required for germination, which is generally around 65°F-75°F for most plants. Covering the containers with clear plastic or using heating pads can help create the ideal conditions for germination.

These are the main concepts discussed in the article. If you have any further questions or need more information, feel free to ask!

Starting Seeds Indoors (2024)
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