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How to Start your Outdoor Vegetable Garden Indoors under LED Grow Lights

Getting a Head Start for your Summer Outdoor Vegetable Garden by Starting Plants Indoors Under LED Grow Lights

Growing an outdoor vegetable garden over the summer can be incredibly rewarding, providing healthy, tasty fresh fruits and vegetables. Starting seeds under vegetable grow lights can help maximize your harvest and allow you to enjoy fresh produce sooner.

Not all garden vegetables are suitable for an early start; plants such as carrots, radishes and peas resent being transplanted and benefit from the cooler soil temperatures of early spring in the garden. Most other vegetables can benefit from being started indoors under a full spectrum LED vegetable grow light, if started at the right time and in the right conditions.

Sowing Seeds Indoors

Different kinds of vegetables should be started at different times to grow optimum-size plants for transplanting into the garden. For the vegetables listed below, the ideal time to plant them in the ground is after the last frost, when the soil has warmed sufficiently. The average date for the last frost in your area can be found here.


For example, if your average last frost date is May 15, as it is in the Boulder / Colorado Front Range area where Black Dog LED is located, you would want to start pepper seeds February 1-14 (January 15-February 1 if you're grafting them), and tomatoes March 15-April 1 (March 1-March 15 if you're grafting).


  • Start with pots or trays filled with sterile media such as a peat-based seedling mix, pre-moistened before planting (this will help to prevent seeds from getting "washed down" too far into the soil).
    • For peppers, tomatoes, eggplant and tomatillos the seeds should be started in small individual pots- 2.5" or smaller, or if sown into flats, the seedlings should be transplanted into small individual pots shortly after germination. Giving the seedlings too large a pot to start makes it harder to properly regulate the soil moisture.
    • For squash and melons, starting the seeds in 4-6" pots is recommended because these plants don't like to be repotted.
  • Small seeds should be planted no more than ¼" deep; larger (squash and melon) seeds can be planted ½" deep.
  • Water carefully, ensuring the soil is not saturated.
  • Using a humidity dome or plastic wrap over the pots to help maintain humidity during germination can be helpful, but be sure to remove this as soon as germination occurs.
  • Place the pots in a warm spot to speed germination. The ideal germination temperatures depend on the type of plant:
    • Tomatoes and tomatillos: between 75 and 80 °F.
    • Peppers and eggplant: between 80 and 90 °F.
    • Squash and melons: between 80 and 95 °F.

Tomato seedlings under a Black Dog LED grow light

Care After Germination

  • Once the seeds have sprouted, move the plants under very bright LED grow lights.
  • Use an oscillating fan to move air around the seedlings. Brisk air movement will help the plants to develop strong, stout stems to prepare for winds when they are moved outdoors.
  • Control the temperature to control the plants' growth.
    • Cooler temperatures will slow down growth and result in shorter internode distances (smaller, stockier plants).
    • Warmer temperatures encourage fast growth and increase internode distances, making plants grow taller.
  • Do not fertilize the plants until they have their first few sets of leaves, and then start with a weak liquid fertilizer. If you're going to be grafting the plants, don't fertilize them before grafting.
  • As the plants grow, they will need water more frequently. When they need watering every day, it is time to transplant them into a larger pot.
    • Slowly step up pot sizes-- don't put them in a large pot right away as the roots will just grow to the outside of a large pot and start circling. Slowly increasing the pot size and repotting several times encourages the roots to use all of the available soil and creates a superior root system.
      • From a 2.5" pot, move into a 4.5" or 5" pot.
      • From a 4.5" or 5" pot, move up to a gallon pot.
      • If necessary to move up from a 1 gallon pot, transplant into a 2 or 3 gallon pot.
    • Unless the plants are grafted, it is helpful to bury the seedling deeper with each transplanting. This will encourage new roots to form on the buried stem, better anchoring the plant against eventual wind exposure and giving a larger root system. Don't be afraid to bury leaves (though it is best to remove them first); as long as the growing tip and a few leaves remain above the soil the plant will be fine.
  • As your plants grow taller, adjust the hanging height of the LED light so that it is always at the appropriate distance above the plants.
  • If the plants begin flowering before they are in the garden, removing the flowers will encourage more vegetative growth and lead to a larger crop when the plants are finally planted outside.

Preparing Plants for Life Outdoors

Your vegetable starts will be acclimated to life indoors and aren't acclimated to conditions outside. A week or two before you're ready to plant them outside, start moving the plants outdoors to a sheltered spot for a few hours a day when the weather is nice. Start with one hour outdoors the first day and slowly increase the time spent outside until the plants are fully "hardened off". If the indoor lighting wasn't bright enough, the plants weren't spaced adequately under the lights, or especially if you weren't using Black Dog LED grow lights with a full spectrum including UV, start acclimating the plants in shade and slowly increase the amount of sunlight they are exposed to each day.

When the time comes, plant your starts out in the garden!

2 thoughts on “How to Start your Outdoor Vegetable Garden Indoors under LED Grow Lights”

  • Vincent V. Grough
    Vincent V. Grough February 11, 2015 at 4:05 pm

    How do I start my lettuce/greens seeds. Can't get any to germinate.

    • Kevin

      Lettuce seeds only germinate well when temperatures are below 80 °F and when they are exposed to light. Lettuce seeds also have a shorter shelf-life than many other seeds, so if your seeds are old they may not germinate well.

      The best way to sow lettuce seeds is to pre-moisten the soil in your pots and sprinkle the seeds directly on top of the soil-- don't cover them with any soil. Put plastic wrap over the top of the pots to keep humidity up, and put the pots under low to moderate levels of light until the seeds germinate. Overheating can be a problem if the light is too bright, or if you're using lights which generate a lot of light plants can't use, so make sure the mini-greenhouse you made with plastic wrap doesn't get too warm. With our LED grow lights, I'd suggest keeping the lights at the recommended vegetative hanging height in a 65-70 °F room until they sprout.

      Once the seeds have germinated, remove the plastic wrap.

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