Starting Seeds Indoors

Starting Seeds Indoors

  • By Cleve Campbell
  • /
  • March 2016-Vol.2 No.3
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Starting seeds indoors is the perfect “fix” for the gardener with cabin fever and a need to get their hands in the soil or simply a desire to get a head start on spring. Despite the frigid temperatures, March is the time to make a seed-planting schedule, to make those final seed purchases, to collect transplant containers and to start seeds indoors for the upcoming gardening season. With a small investment, and a bit of space and care, you can grow healthy transplants that are already to go into the ground outdoors when the proper time arrives.

Why start seeds indoors?

  • Growing transplants from seed gives the gardener more choices than are normally available in gardening centers or other retail outlets.
  • By growing your own transplants, you can control when you plant them outside.  There’s no need to wait until they are available in a retail outlet or to wait for a mail order supplier to ship on a predetermined date.
  • Indoor seed starting definitely saves you money.   You might not reap these savings at first  — if you have some first-year-set up costs.
  • Growing your own transplants increases your garden’s output.  How?  Well, you get a 2- to 8-week head start by starting seeds indoors rather than sowing seeds directly into the garden, which allows you to get an earlier harvest and makes for a long harvest season.  Besides, sowing seeds directly in the garden is not always practical for some crops. For example, tomatoes and peppers cannot be planted until the last frost and after the soil is warmed. If seeded in the garden at that time, tomatoes and peppers need more than a 100 days (including germination time) to produce the first fruit, In addition, newly emerged seedlings are very tender and easily killed by insects or disease or shaded by quicker-growing weeds. Starting your seedlings indoors allows an earlier start in the garden, resulting in earlier yields of certain crops, and allowing for better use of limited garden space.
  • You control the environment.  If you want organic plants, you have control over the soil, compost, and inputs like fertilizer.

What seeds should you start indoors?

The chart below was adopted from the Virginia Cooperative Extension Publication 426-316 and provides a list of plants recommended for starting indoors.


Plant Production Chart
Crop Days to Emergence Optimum Germination Growth Range (º F) Number of Weeks to Transplant
Broccoli 3-10 65-85 º 5-7
Cabbage 4-10 50-85 º 5-7
Cauliflower 4-10 50-85 º 5-7
Celery 9-21 50-65 º 10-12
Cucumber 6-10 65-85 º 4 (peat pots)
Eggplant 6-10 65-85 º 6-9
Lettuce 6-8 50-65 º 3-5
Melons 6-10 65-85 º 3-4 (peat pots)
Onion 7-10 65-85 º 8
Parsley 15-21 50-85 º 8
Pepper 9-14 65-85 º 6-8
Squash 4-6 65-85 º 3-4 (peat pots)
Tomato 6-12 65-85 º 5-7


The first general rule is to start with good seed.   Buying seeds from a reputable supplier will help to ensure good germination. If you buy at your at your favorite garden store, check the date on the seed package to ensure that the seeds are fresh.  As seeds get older,  the germination rates decrease.

Factors Affecting Germination

The factors that affect germination and seedling growth are moisture, temperature, oxygen, and light.

Moisture (water): The first step in the germination process is the seed’s absorption of water. A continuous supply of moisture without fluctuation in the growing medium is important to ensure germination.

Temperature (heat): Some seeds will germinate over a wide range of temperatures, whereas others require a narrow range. Many seeds have minimum, maximum, and optimum temperatures for germination. For example tomato seeds have a minimum germination temperature of 50º F and a maximum germination temperature of 95 degrees F, but an optimum temperature of above 80 degrees F. How fast seeds germinate is affected greatly by soil temperature.  For example sweet pepper seeds will germinate in 9-14 days when the soil temperature is in the optimal range of 65-80º F.  At 55-65º  the time to emergence is 21-28 days.

When germination temperatures are listed on the seed packets or in seed catalogs, they are usually the optimum temperatures unless otherwise specified. Generally, a range of 65ºF to 75ºF is best for most plants. This often means that your germination flats may have to be placed in a heated area or on heating mats to maintain that optimum temperature. Remember this is soil temperature, not ambient or air temperature.


Seedling trays with heat mats Source: University of Vermont

Seedling trays with heat mats
                    Source: University of Vermont

Oxygen: All viable seeds breathe. Their respiration rate increases dramatically during germination; therefore, the growing medium must be loose and well-aerated. If the soil is too wet or compacted, the oxygen supply during germination is reduced, and germination can be severely retarded or inhibited.

Light: Light can either stimulate or inhibit germination, depending on the plant. Although most seeds germinate in the darkness, a few plants, such as lettuce and parsley, require light, whereas others, such as tomato and peppers, germinate best in the dark. Seed catalogs and seed packets generally indicate light requirements. I always take a minute to read the seed packet or seed catalog to determine the light requirements for germination.

Transplant Containers: You can use any type of container which will hold a soil mix and are about 3 inches in depth, and capable of holding a minimum of 3 tablespoon of soil.  The container must have holes in the bottom for drainage. There are lots of possibilities for containers —  recycled yogurt cups, cottage cheese containers, cut off milk cartons, discarded aluminum trays, and recycled cell packs. New cell packs, peat pots, or peat pellets can be purchased at garden centers, farm supply stores or hardware stores, and are usually (in season) available in large box stores or online sources. I have a gardening friend that creates his own transplant containers from strips of newspaper. Peat or paper pots that break down in the soil are particularly good for raising seedlings that do not transplant easily —  such as cucumbers, squash and watermelon.

If you are recycling containers that have been previously used for growing plants,  be sure to sterilize them before reuse. Wash them thoroughly with hot water and soap and then soak for 10 minutes in a solution of 1 part household bleach and 9 parts water. After sterilizing them, rinse well and let dry before use.

If you are using recycled non-plant containers such as plastic cups, remember to punch or drill a hole in the bottom of the container for drainage.

Holes punched in plastic cups for drainage.

Holes punched in plastic cups for drainage.

Growing Medium

A good germinating medium should be fine and uniform, well aerated and loose. The medium should be free of insects, disease organisms and weed seeds. It should also be of low fertility and capable of holding and moving moisture.

Artificial soil-less mixes offer all these desired qualities. The basic ingredients of such mixes are usually shredded sphagnum peat and fine grade vermiculite, both of which are generally free of  weed seeds and insects. Avoid using  plain garden soil or regular potting soil, as it tends to crust and harden, making it difficult for delicate seeding to “break through.” In addition, ordinary garden soil may contain weed seeds and diseases that may contribute to damping off.

Once the seedlings have emerged and developed one or two sets of true leaves, you can transplant them into a slightly larger container with a coarser seedling-growth mix.

When to sow your seeds

You have rounded up all your material: containers, soil and seed.   The next thing to be determined is when to plant your seeds. The most critical date to keep in mind is the last average frost date in your area. In our area in central Virginia, it is May 10th – 15th.   For most crops, that date is used in calculating your indoor seed-starting date.

Let’s look at tomatoes, for example.  Tomatoes cannot be set outside until the last average frost, May 15th. Tomato seedlings require about 5-7 weeks after sowing to be ready for transplanting, so counting back 6 weeks from our average frost date of May 15th,  I arrive at my targeted date for indoor sowing —  around April 2nd.

It’s a different story with certain so-called “cold weather” crops, such as cabbage and broccoli.  For example, in our area we can start to transplant into the garden cold weather crops such cabbage around the second week of April, i.e., April 15.  If I know that cabbage seedlings require an average of 5 to 7 weeks to be ready for the garden, I assume about 6 weeks indoor growing time.  Then I count backwards 6 weeks from April 15th and come up with March 10 as my targeted date for sowing cabbage seeds indoors.

The varying timings required by different crops could give you a headache.  For this reason, I have found that making a schedule for different crops can be a very useful tool.

There are many online sources that provide information on temperature and time requirements for growing transplants. Remember, these are only guidelines. In general, the length of time is based on optimum germination temperature. You may need to adjust the time requirement to fit your specific growing environment. Also, the charts are guidelines, and there are always exceptions. For example most charts suggest 6-8 weeks to grow pepper transplants, and I have found that works well for most peppers; however, if you are into growing hot peppers, 6-8 weeks usually is not enough, especially for capsicum chinense pepper types. Chinense varieties include super hot peppers such as Habanero, Scotch bonnet and Bhut Jolokia (Ghost). These super hot varieties,  because of their long germination periods and slower growth rates, can take anywhere from  8-15 weeks to grow transplants, again depending on the various environmental conditions. Over the years I have learned to read the seed packets for instructions on growing times. The information on the seed packets is a must-read.

 Steps in Planting Seeds Indoors

Now that you’ve figured out when to plant your seeds, you can start sowing seeds.  Remember that the containers for starting seeds should be sterile and free of harmful chemicals.

  • Pour the germination medium into a clean bucket or small tub.   Add water to the germination medium so that it is saturated like a sponge. If you squeeze a handful of the medium and water runs out it is too wet, so add more medium.
  • Fill the container to within ¾ inch of the top with the growing medium. Make sure the container has adequate drainage.
  • Add two to three seeds per pot or cell, unless the seed is old or has a low germination rate.  In that case, add a few more seeds.
  • Cover the seeds with the germination medium or horticultural vermiculite. How much to add?  Follow the depth instructions on the seed packet.
  • Label the containers.   It’s useful  to note when you planted the seeds and how long they took to sprout. Keep a journal of what you did and when you did it. Your observations will be critical in fine-tuning your planting strategies and schedule in the years ahead.   That’s the way to achieve success in producing vigorous, sturdy, short, dark green transplants.
  • Water each container very lightly to ensure the seeds have made contact with the soil. A misting bottle works well.
  • Cover the container(s) with plastic or cling wrap and then place in a warm area or on a heat mat. This keeps the seeds warm and moist to increase germination rates.
    Plastic cover over planting cantainer

    Plastic cover over planting container

  • Each day check for germination.  Once the seeds have sprouted, immediately remove the plastic wrap or plastic dome and move into strong light. A south window sill is a good place, but remember to give the pots/containers a  turn each day so the plants grow straight instead of bending towards the light.  The use of fluorescent lighting is recommended to avoid spindly plants. If you are using fluorescent lighting, position the lights 3 to 6 inches above the seedings. Closely monitor the plants, and adjust the light up as the seedlings increase their height. The lights should be kept on 12 to 16 hours per day. Regular light bulbs or incandescent bulbs are not recommended because they produce too much heat in relation to the light given off. They also lack the blue spectrum light that keeps seedlings stocky and dark green. Keep the soil moist and maintain air temperature of 65-70º F.

Watering and Damping Off

Keep the  soil moist while the seeds are germinating.  I recommend a a spray bottle, which allows you to mist the surface gently without washing away the potting mix.   If the containers are sitting in a tray, you can simply add water to the tray, where it will move upwards into the growing medium.  Be sure to drain excess water that remains or accumulates in the tray  to keep the roots healthy. The planting medium should be kept moist, not wet.  If the soil is too wet you run the risk of retarded root growth that often leads to disease problems, such as damping off.

Damping Off  is a plant disease caused by several fungi, including molds such as Phtophthora, Pythium, Fusarium and others. These fungi occur in all soils and are water-loving organisms that thrive in wet, cold, or poorly drained soils.

Size does matter — seedlings should not be started in large pots, because large containers hold more water, more than the small seedling can use, making an ideal environment for damping-off. It’s recommended to start your seeds in small containers first, and transplant them into larger containers once the seedlings have germinated and developed.

A fungus in the soil can attack seeds and seedlings as they begin to germinate and grow. Pre-emergence damping-off occurs when the seed or seedling dies before it reaches the soil’s surface, whereas post-emergence damping-off occurs after the seedling emerges and grows to a height of an inch or two, causing it to wilt, fall over and die. Plants that are attacked by these fungi but do not damp-off are often stunted. A constricted stem at or just below the soil line is a sign that the plant underwent a fungus attack.

Damping-off is controlled primarily through good sanitation, high quality planting material, and proper cultural and environmental controls. Damping-off is worse when soil is wet or compacted.

Damping-Off Source USDA

Damping-Off (Rhizoctonia) is a fungal disease of seedlings that girdles the plant stem where it enters the growing medium, causing it to topple over. 
Source: USDA


Thinning Out

As soon as your seedlings have developed at least one set of leaves, you need to provide them more room. If you have planted them in individual pots or cell packs you can take a pair of sharp scissors and simply snip the smaller plants, leaving only one plant, the strongest and healthiest plant. A second option is to gently separate the small plants with a clean knife or plant label. My favorite tool for separating seedlings is a small cocktail fork. Gently ease the seedlings apart, being careful to avoid tearing the roots in the process.  Then repot the seedlings in a slightly larger pot. Handle small seedlings by their leaves; those small, thin stems break easily. Failure to thin or transplant crowed plants can result in spindly seedlings that may not develop properly,


The seed will provide sufficient nutrients until the seedling develops its first set of true leaves. The true leaves are a sign that the seedling has used up its store of nutrients in the seed and will require outside fertilization. Once the first true leaves appear, water with a half-solution of fertilizer; you can use a water-soluble all purpose plant food or organic fertilizer such as fish emulsion. Fertilize only once a week. Water as needed the rest of the week. As the seedlings grow, gently brush them to and fro with your hand to “mimic a gentle breeze.” This will help strengthen the stems and prevent excessive stem elongation (leggy plants).

Dipiction of true leaves Source:USDA

Depiction of true leaves


Before transplanting seedlings into the garden, it’s important to condition the plants for outdoor conditions. This conditioning is achieved by hardening the plants. Hardening is the process of gradually acclimating tender plants to the outside environment. Harden the plants two weeks before transplanting by moving them into a shaded area outdoors such as a porch or under a shrub. Then, move them gradually to sunlight for a short time during the day. Slowly increase the length of exposure time.  Do not expose the seedlings to freezing temperatures or strong winds. Reduce watering, but do not let the plants wilt.  After proper hardening, carefully transplant the plants into the garden.

Seeds have their individual needs.   Always follow the instructions on the seed packets, as that is your recipe for success. Thanks for joining us in the Garden Shed. We hope to see you again next month.


USDA, Community Gardening Guide, “Season Extension,”

“Plant Propagation From Seed,”  Va.Coop.Ext. Publication Number 426-001,              

“Seed for the Garden,” Va. Coop. Ext. Publication Number 426-316         

“Capsicum Chinense Pepper Species,” grow-organic-vegetables/capsicum-chinense-pepper-species

Cornell University, Planting Options                                 

“Damping Off Diseases in the Garden,” University of California Agriculture & Natural Resources,

Seed Savers Exchange, Habanero Pepper (Capsicum Chinense)                                      

“Back to Basics: How to Start Seeds indoors,”   Texas Coop.Ext.,                                                                                                  aggie-horticulture/2002/may02

“A Practical Guide to Using Mechanical Stimulation to Prevent Stretching in Tomato and Cucumber Transplants,” Cornell University,








  1. Katie Benton

    Great and very helpful post! It’s my sister’s first year of gardening and she decided to start with buying all the transplants she’ll need for this season. I’m doing my seed starters few years already and offered to prepare extra of each kind for her, but she had ordered some already. I’m definitely recommending your post to her, so she would know what to do and what not in future. Thank you for the great information!

  2. lewis

    I love how this article shows the proper way of planting seeds indoors. And by doing this, I have the power over the result of the seeds I planted. And that is to produce high-quality fruits and veggies. Good job!

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