Planting Seeds Indoors

Author: National Gardening Association

Watching seeds sprout and grow is an exciting and magical experience for young gardeners, and presents a series of engaging questions: When will the sprouts appear? How did those tiny seeds transform into leafy-green plants? How much will the seedlings grow each day?

In addition to sparking inquiry and wonder, seed propagation provides numerous teaching opportunities from identification of seed and seedling parts to investigation of more complex processes, including germination and photosynthesis. Planting a bean or marigold seed in a paper cup and watching what happens is a popular classroom activity, and for many children it’s also their first gardening experience. 

However, starting seeds can be a challenge in indoor environments. To help make your classroom seed-starting adventure successful, we’ve compiled a list of frequently asked questions related to indoor growing. Click on one of the following links to jump directly to that question:
 
General suggestions for indoor seed starting
Systematic way to start seeds indoors
Helpful hints for indoor seed starting with very small children
Homemade containers for seed starting
Ideal temperature for starting seeds indoors
Fast-germinating vegetable and flower seeds
Growing plants from seeds collected in the wild
Troubleshooting for seeds that don’t sprout
Troubleshooting for seedlings that die
Expanding the basic “this is a seedling” lesson 
Developing a seed-sowing and -planting calendar

Question: Do you have general suggestions for starting seeds indoors?
Answer: Each type of seed has slightly different requirements, but the basics are the same:

• Use a commercial seed-starting mix, which is sterile and provides proper drainage. Avoid using garden soil, as it can introduce disease-causing organisms.
• Follow the recommended planting depth on the seed packet when sowing seeds. This is generally 1-1/2 to 2 times the width of the seed.
• Maintain even moisture. Seeds must stay moist in order to germinate properly.
• Good light is essential. Once you see signs of germination, move the seed trays to a light-filled area. Seedlings need full sun from a south- or west-facing window, or fluorescent lights positioned two inches above the foliage. Use a timer to keep fluorescent lights on for 12 to 16 hours a day.
• Seedlings like to be warm. Place the containers in a warm spot (70 to 75 degrees F) or on top of a heat mat.
• Keep the soil moist, but not overly wet. At first, lightly mist the soil. Once seedlings emerge, place the containers in a tray and water from the bottom to encourage deep root development. Check daily and do not let the soil mix dry out.
• Wait to fertilize until the seedlings’ true leaves (the second set) appear. Then use a weak solution of a high-phosphorus fertilizer (e.g., 5-10-5), which will help the seedlings develop a strong root system. When the seedlings have one to three sets of true leaves, transplant them to a larger container.

Question: Is there a systematic way to start seeds indoors with my class?
Answer: First, mix and moisten enough potting soil for all your containers. If possible, use containers that are all the same size. This will help standardize seeding, watering, and transplanting. Fill the containers with exactly the same amount of the soil mixture. 

Have just one type of seed per planting station or per session so that different plants do not get combined in any one pot. Once seeds are planted, it is very hard to tell what went where!

Remember to have a labeling system ready when you plant the seeds. One method is to write the plant name and date, etc., on good-quality masking tape and stick that to the outside of the container. This label usually lasts quite well. You can also write on a Popsicle stick with an indelible marker, then push the stick down the side of the container.

Finally, keep a log of what you did so you can repeat it (or not!) next time and so you will have a cumulative record of your activities for future reference.

Question: Do you have any helpful hints for very small children planting seeds indoors?
Answer: It’s nice to show children a mature plant or photos of mature plants before you sow seeds. If possible, also show them a seedpod or seed head from the actual plant you are growing. Providing this link between plant and seed adds to the child’s understanding of the planting ritual and the cyclical nature of gardening.

Young children have tiny fingers and may have an easier time with small seeds than an adult might. A child’s “pinch” of fine seeds can actually be quite small! Some young children delight in carefully counting out two or three small seeds and putting them in each pot or cell. Other young children do better with larger seeds, such as bean, melon, squash, or marigold. If you’re using a mix of larger and smaller seeds, try assigning the seeds to children based on your gut feeling about each child’s skill set.

There are several techniques to help children space seeds evenly when planting. For starting seeds in a tray, draw a light line in the potting soil with a pencil or stick and then ask children to space the seeds along that line as far apart as their thumb is wide, or as far apart as a specific spoon is wide. For small pots, instruct children to place one seed at the top, one at the bottom, one on the left, one on the right, and one in the middle for luck. All children love decorating labels for their own pot of seeds, but you can add their (legible) initials, too!

Question: What common things can we use to make our own seed-starting containers?
Answer: Different plants need different-sized containers, but most do reasonably well in 6-inch pots. You can use smaller pots to germinate seeds, but you'll need to transplant the seedlings later. To keep it simple, use the same 6-inch container for germinating and growing. Here are a few container ideas:

• Milk cartons – Cut quart containers down to make 4-inch by 6-inch containers, or have kids collect pint-size cartons from school.
• Plastic soda bottles -- Two-liter bottles make nice deep containers. Cut off the tops to make 6-inch planting pots.
• Yogurt cups -- The six- or eight-ounce cups are ready-made seed-starting pots.
• Egg cartons -- Egg cartons work well for starting seeds, but you'll have to transplant them to larger containers as soon as they’ve sprouted.
• Old shoes, hats, baskets, etc. – You can use anything that can hold soil, as long as it has drainage.

Important: No matter what your homemade container originally was, you must first clean it thoroughly and add a drainage hole in the bottom to avoid waterlogged plants. Drainage holes are usually about the size of a dime.

Question: What is the ideal temperature for starting seeds indoors?
Answer: Most seeds have an optimum temperature range for successful germination. Indoor germination temperatures are slightly warmer than you’ll need for growing mature plants. As in most gardening adventures, general recommendations go hand in hand with some amount of experimenting. For starting seeds, aim for a daytime range of 60 to 80 degrees F and a nighttime temperature that’s 10 to 15 degrees cooler. (Temperatures should not exceed 95 degrees F or drop below 40 degrees F.) If you feel comfortable, chances are your seeds do, too. You don’t need to closely monitor temperatures for different plants, but keep in mind that different seeds do germinate at different minimum, optimum, and maximum temperatures. (Refer to the seed packet for specific information.)

Question: Since kids can be impatient, what are some fast-germinating vegetable and flower seeds we can grow?
Answer: Flower seeds that germinate quickly include bachelor's buttons (7 to 10 days), sweet William (dianthus, 5 to 10 days), gloriosa daisy (rudbeckia, 5 to 10 days), ageratum (6 to 10 days), cosmos (5 to 7 days), sweet alyssum (8 to 15 days), zinnia (5 to 7 days), and Mexican marigold (5 to 7 days).

Vegetable seeds worth trying include corn (5 to 7 days), cucumber (7 to 10 days), lettuce (7 to 10 days), and watermelon (5 to 7 days). To ensure success, plant your seeds in moistened seed-starting mix and place the containers in a warm area, such as on top of the refrigerator.   

Question: My children want to try growing plants from seeds they’ve collected from the wild. What do we need to know to be successful?
Answer: It’s exciting but can also be challenging to grow plants from wild-harvested seeds. Seeds of many plants have adaptations that ensure dormancy until conditions are right for successful germination. This fact can inspire young gardeners to ask questions and conduct investigations. Here are the basics.

Some seeds require a period of cold temperatures (winter) followed by warmth (spring) to germinate. Others have hard seed coats that soften up during freeze-and-thaw cycles. If you're not planting the seeds right away, dry them in a room with good circulation and then store them in a cool, dry place. Some seeds may germinate without treatment; try soaking some seeds overnight in water and then wrapping them in a wet paper towel and placing the towel in a plastic bag for 5 to 7 days.

To break the dormancy of a seed with a hard coat (such as acorns, honey locust, lupine) you’ll need to make an opening in the hard shell that allows air and water to reach the embryo. Use a file or sandpaper to lightly scar the shell before planting it. Boil lupine seeds then let them soak overnight in the water before planting. Seeds collected in the fall may require a chilling period for successful germination. Place the seeds in moist sand in a covered glass jar, or sow them directly in pots filled with a soilless mix and wrapped in plastic. Keep the seeds in the refrigerator for one to four months. Periodically check to make sure they are staying moist.

Sow tree seeds at a depth of three times the diameter of the seed, and wildflower seeds about the same depth as the seed diameter. Press very fine wildflower seeds and grass seeds into the soil and don’t cover them. Remember to provide light, warmth, and moisture while the seeds are germinating. After two to four months of growing indoors (or when spring arrives), you can transplant the seedlings outside.

A word of caution: Never harvest seeds of plants you know to be endangered or in short supply in your area!

Question: Many of the seeds my kids started indoors last year never sprouted, what did we do wrong? We bought commercial potting mix and containers and planted the seeds according to the packet directions. We kept the room between 70 and 75 degrees F and watered faithfully.
Answer: Please don't give up. Gardening indoors has a few challenges and many rewards. The seeds might have been over watered — drowned seeds are a common occurrence with eager young gardeners. Misting the soil gently provides seeds with enough water to germinate without becoming waterlogged.
Seed germination may be another factor, and in addition to keeping seeds moist and maintaining a constant temperature, you might also want to test their viability before planting. Use the "wet paper towel" test to ensure that the seeds you plant will actually sprout. Take 10 seeds from each type being tested and lay them on moist paper towels (one seed type per towel). Fold up each moistened towel and place it in a labeled plastic bag. After 7 to 10 days unroll the paper towels and count the number of seeds that germinated in each towel. If fewer than five of the 10 seeds in a towel sprouted, buy fresh seeds.

There are also a few other explanations for why your seeds may not have germinated:

• The soil temperature was too cool or too hot. Check the seed packet labels for recommended temperatures.
• The soil mix dried out. Make sure the soil mix is evenly moist before you plant the seeds, and keep it lightly misted during germination. If you keep the containers covered with plastic or wax paper, the soil mix will stay warm and moist.
• The seeds were planted too deeply. Most seeds should be planted at a depth of three times the diameter of the seed. Sow very fine seeds on top of the soil mix and pat down gently.

Question: Our young gardeners were heartbroken when newly emerged seedlings died. What might have happened?
Answer: When newly germinated seedlings appear healthy and start to grow, then wilt unexpectedly, the cause is probably a fungal disease called damping off. This fungus attacks the stem near the soil, weakening it and causing it to rot. In addition to killing seedlings, damping off can sometimes prevent seeds from germinating.

Damping off can occur when seed containers are reused without first being sterilized, or when a nonsterile potting medium (such as garden soil) is used. Overwatering and overfertilizing with nitrogen also contribute to the problem, as does poor air circulation.

To prevent damping off, use new soilless potting mix, sterilize containers with a bleach solution (1 part bleach to 9 parts water), keep soil evenly moist but not wet, and wait to apply fertilizer until seedlings have their first set of true leaves. Also, thin seedlings so they aren’t crowded in the container.

If damping off occurs, remove infected seedlings and surrounding soil at once. Check the soil moisture and, if needed, allow it to dry out slightly. If seedlings are crowded, thin them to promote air circulation. If you need to replant, sterilize the containers and utensils, and use fresh commercial seed-starting soil.

Question: The kids have had the basic “this is a seedling” lesson since preschool; what else can we teach them?
Answer: Despite their common characteristics, plants have a seemingly infinite number of variations. Children love to discover “oddities,” so try starting a wide selection of seeds and examining the resulting seedlings.

Begin introducing botanical concepts, such as alternate and opposite leaves, so you can compare and contrast the plants easily. See if the children can predict an unknown plant's ultimate identity and size based on the seedlings' growth at various stages. See if they can match the seed to the fruit or flower from which it came. Look at the seedlings in terms of leaf texture, shade of green, overall form, and growth rate. Can children recognize plants that are related?

Grow a few extra plants. Once established, pull them up and wash off the roots. For example, look at greens, radishes, potatoes, carrots, onions, parsley, and chives. Examine the underground parts of flowers, such as tulips, bearded iris, cannas, marigolds, and daylilies, too. Look for similarities and differences.

Try starting some seeds that germinate over a long period of time, such as tree seeds or perennial seeds, as well as your usual assortment of vegetables and annual flowers. You can also experiment with pre-germinating seeds and testing the viability of older seeds. Finally, try growing some seeds that need cold stratification in order to germinate. Kids get a kick out of putting seeds in the refrigerator for a few weeks!

Question: How can we develop a seed-sowing and -planting calendar with our kids?
Answer: This is a great opportunity to discuss geography, climates, and different plants' needs. First, get a monthly calendar with big blocks for the days. Next, you'll need the seed packets and a good idea of your region's last frost date (for transplanting the plants). If you don't know the last frost date, check with your local Cooperative Extension agent.

The dates for starting the seeds are based on how long they take to germinate, how much time they need to become established indoors, and when they can be transplanted outside. Different seeds sprout at different rates. Cool-weather plants (broccoli, pansies) can be transplanted outside two to four weeks before the last frost date. Medium-cool-weather plants (lettuce, cosmos) may be transplanted on the last frost date, and warm-weather plants (tomatoes, impatiens) should be transplanted after all danger of frost is past.

Coordinating the germination rate, growing time, and transplanting date will ensure that your plants will be ready, willing, and able to move into the garden. With a calendar in hand, count back from your last frost date to how long the seeds take to germinate and the number of weeks needed for indoor growing time.

For example, a cool-weather plant such as broccoli may be transplanted two to four weeks before the last frost date and requires six weeks' growing time before being transplanted outside, so count back a total of eight weeks from the last frost date. You'll end up with different dates for starting the seeds of each plant and transplanting them outside. The kids could count and label each of these days on the calendar.

Need more information?
Click here for step-by-step instructions for planting seeds.