University of Maryland Extension

Starting Seeds Indoors

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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 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 (see “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 in. tall 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 is above the soil.

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

seedlings under Lightstand

You can grow two standard size (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

Choose containers and prepare growing medium

  • Choose your containers 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.

dry media sitting in wheelbarrow
Dry media in wheelbarrow
wet media
Add water

mix water media
Mix thoroughly

fistful of moist soil
Moist soil ball

Fill containers

  • Fill your container to within ¾ of an inch from the top with the moistened medium. 

adding growing  media to container
Add media to container
filled flat
Black flat filled with potting media

  • 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

furrows
Mark rows in flat with straight edge

  • 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

hand seed
Planting seeds by hand
index card
Planting seeds using folded index card

covering seed with growing media
Lightly cover seeds with media

  • 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. 

spacing
Top 2 rows correctly spaced

thinning
Hand pull seedlings to correct spacing

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.

seed packet
Planting information on back of seed packet

seedling growing in yogurt cup
Arugula seedling in yogurt cup

young broccoli
Broccoli seedling in 4-cell pack

rows of seedlings
Lettuce seedlings planted in rows 

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
Broccoli85 to 1070Either
Cabbage85 to 1070Either
Cauliflower85 to 1070Either
Cucumber4 or less5 to 1085Either
Eggplant85 to 1070Either
Lettuce85 to 1070Light
Muskmelon4 or less5 to 1085Either
Pepper85 to 1080Either
Squash4 or less5 to 1085Either
Tomato65 to 1080Either
Watermelon65 to 1070Either

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