Seed Starting Indoors- An Introduction


There are a number of good reasons for starting seeds indoors. As a student worker, I planted many seeds for greenhouse growing at my university. Hands on learning with professionals in such an ideal setting was a great foundation. Fast forward a few years and I found myself looking at nurseries for transplants of a particular cosmos. It was no where to be found, so my number one reason to start seeds indoors is to have flower and vegetable varieties that I cannot source. But there are 3 more reasons I do indoor seed starting.

Reason two is the seed germination rate is higher in a controlled environment than direct sowing outdoors. As a passionate flower lover, I want all the flowers I can possibly grow.  After sowing seeds into borders and beds, there is a watch and wait time to see what germinates. By the time, some seeds germinate if the rate is low it may be too late to sow more seeds.

Also, starting seeds is budget friendly gardening. Granted there is some cost for initial materials that not everyone can manage but purchases can be spread out over a few seasons. Once you have the essential equipment, it lasts many seasons.

And, the fourth reason to start seeds indoors is timing. It gives a head start on the growing season. This is particularly important in warm climates like central Texas where I garden. It is a special advantage for the vegetables that need warmer temperature to flower and set fruit – crops like tomatoes, peppers, eggplant and cucumbers. If started indoors, you can have more mature, larger transplants for planting into the garden at the earliest possible time. There is a bit of a learning curve for indoor seed starting, but there are loads of resources available. And, like many other gardening techniques, you learn as you go and have growing success.

Starting seeds indoors is also fun and rewarding. Here are the basic steps for how to grow plants from seed indoors.

Indoor seed starting kit

For starting seeds indoors, you will need a seed starting setup – a cell tray kit, heat mat, lights and seed starting mix. Shop all my favorite supplies in my seed starting essentials guide here.

How to start seeds indoors?

Fill sterilized, plastic cell trays with seed starting mix that contains no soil. If using traditional mix, moisten it first. Try mixing it with water in a large clean container then fill the cells. The mix is difficult to wet and is messy so I found what I think is a better mix, Miracle-Gro Cactus, Palm, and Citrus Mix.  It is slightly easier to work with and gets good results. Fill the cell trays with mix, moisten then let sit and repeat this process until the mix is thoroughly moist.

How to grow plants from seed step by step

Next sow 2 to 3 seeds per cell. Read seed packet directions, but I almost always plant 2 seeds. Plant seeds at the depth specified on the seed package. Some seeds need light to germinate and are placed on top of the mix. A very thin covering of vermiculite may be sprinkled on top, then mist with a spray bottle. If the seeds can be planted deeper, it is easy to just place them on top of media, then add mix on top and mist with water. Be sure to label each tray.

Put the cell tray in the holding tray and cover with the dome. A heat mat is how to germinate seeds quickly and effectively. Depending on seed needs for germination, place the tray on a heat mat on a shelf with lights hanging a few inches above the tray. Once seeds germinate, add water to the bottom tray, just enough for the cell tray to absorb or wick up in a day or less.


  • Water only via the bottom tray, to deter disease. Add water to the bottom tray when it is dry.
  • Never allow the seed tray to sit in water for more than a day. Check it daily.
  • Seeds need 14-16 hours of light daily. Light should typically be just a few inches above the seed trays.
  • An outlet timer is convenient to operate multiple sets of lights automatically.
  • Remove the heat mat and the cover when almost all the seeds in a tray have germinated.
  • Plant the same type seeds in a tray because germination rates vary.
  • Rotate trays daily because the outer edges get less light.
  • Materials must be clean, washed with soap and sterilized. Use a solution of 1 part bleach to 10 parts water to deter disease.
  • If seedlings get leggy, they did not get enough light. They will fall over and may not recover.
  • A fan or gentle breeze strengthens seedlings.

Thin, fertilize and pinch

Use sharp tipped pruners or scissors to thin seedlings to one plant per cell. Fertilize when seedlings have their second set of true leaves. Use a liquid, water soluble product at half or quarter strength. A natural product like fish emulsion is good but any nitrogen source works. Some seedlings like cosmos benefit from pinching back or snips can be sued to make seedlings sturdier. Place a fan on seedlings once they have 2 to 3 sets of leaves to strengthen stems.

Potting up and hardening off

When roots are growing out of the bottom, pot up seedlings into an individual pot about 2-3 inches. Use fresh potting mix. Place them in a tray for carrying and continue to bottom water.

Hardening off is at least a week-long process of acclimating seedlings to outdoor conditions prior to planting outdoors. Move seedlings outside into shade during the day beginning with a couple hours and increase the time each day. The last two days place them in bright shade. Always bring seedlings indoors at sunset. Do not leave them out in rain or high wind.

When to start seeds indoors

Timing is everything in gardening. When to start seeds varies depending on the plant and your location. Seed packages may indicate a range of weeks before average last frost. Others may give no directions while some say to direct sow into the garden.

Knowing your average first and last frost date is essential to successful seed starting. Find those dates by searching with your zip code. My last average frost date is March 1 and average first frost is November 30. Read seed package directions and simply count backwards to identify the window of time that it is best to start seeds indoors. For example, cosmos says start seeds indoors 4 to 6 weeks before average last frost date, so I can start seeds indoors as early as January 21. Basil says start indoors 6 to 8 weeks before average last frost, so I can start these seeds as early as January 7.

There are a number of vegetables and flowers that can be grown twice a year. In my Texas zone 9a, broccoli transplants can be set out mid-January through February and also mid-September through early November. The growing season in warm climates like Texas can be long, which means there is a long period of time to start seeds indoors, allowing for multiple sowings and plantings.

Research the plants you want to grow to learn when to seed indoors. Know the last and first average frost and freeze dates for your location. Also, a soil thermometer is a handy tool because soil temperature is a factor for when cool and warm season plants thrive outdoors.

I’ve enjoy seed starting indoors on my own. I have learned a lot doing it for myself, but as I mentioned, I had some previous experience in my job as a student worker at the Texas A&M greenhouses was to mix the seed starting media and prepare seed trays for the head propagator. I learned even more when I taught plant propagation to Master Gardener classes. Master Gardeners are some of the most dedicated propagators I know.

If this is your first time to start seeds indoors, it is a good idea to start small with just a few varieties then grow more with your success. Relax, enjoy the experience and you’ll get better and more confident each season.

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