Starting Vegetable Seeds Indoors

The timing of seed sowing depends on the recommended planting date for a specific crop, the speed of germination, and the rate of transplant growth. The following is a guide for the approximate amount of time needed to grow transplants indoors before they are transitioned outside:

• Leafy greens: 2 to 4 weeks
• Cabbage, broccoli, and cauliflower: 4 to 6 weeks
• Cucumber, squash, and melons: 3 to 5 weeks
• Tomato: 5 to 6 weeks
• Pepper, eggplant, and onion: 8 to 10 weeks

A common mistake is to sow seeds too early, resulting in tall, root-bound plants that do not perform well in the garden. Use fresh seed or properly-stored seed from last year. Surplus seeds should be stored in a cool, dry place, such as a refrigerator. (Storage at room temperature may result in a shorter viability time than chilled storage. A freezer can work as long as the seeds are dry enough for storage and don’t freeze and thaw repeatedly.)

You can also stagger sowing times to produce midsummer, late summer, and early fall transplants to keep your garden productive. It’s difficult to find vegetable transplants in stores beyond mid-June.

Light is measured by its intensity (closeness and brightness), duration (length of time the light is available, also called a photoperiod), and quality (including blue and red wavelengths within the visible color spectrum).

Natural light from a window is seldom enough for strong seedling growth. Plant stems usually stretch and lean towards the light, which can result in weak plants (having poor vigor and sturdiness). Acceptable plant growth usually only occurs in south-facing bay windows or sun rooms that receive ample direct sunlight.

“Grow light” is a general term for any light fixture specifically designed to promote plant growth. They typically use fluorescent or light-emitting diode (LED) lamps. Although human eyes are most sensitive to light intensities in the yellow-green portion of the light spectrum, plants mainly utilize the red and blue portions of the spectrum. Light sources that look relatively white to our eyes might have different amounts of light in each part of the spectrum, which can affect plant growth.

Broad-spectrum LED grow lights that mimic daylight, and fluorescent light fixtures with cool-white tubes (6500K color temperature), are good options for growing transplants indoors. More expensive full-spectrum fluorescent grow lights produce a balance of “warm” (redder wavelengths) and “cool” (bluer wavelengths) light, which enhances foliar growth and produces thicker stems than cool-white fluorescent tubes and which is needed for producing flowers on indoor plants. Some gardeners use one warm-white tube and one cool-white tube in a two-tube fixture to gain the same effect. Similarly, LED grow lights may combine warm-white and cool-white diodes in the same fixture.

Learn more about grow light options and how to interpret the technical specifications listed on the package in the four-part Maryland Grows blog series: An Introduction to Gardening Under Lights

Make your own light stand

You can make a lightweight polyvinyl chloride (PVC) light stand that can be taken apart and stored when not in use. The pieces fit together snugly and securely. If desired, you can use PVC primer and cement to seal the joints for a light stand that cannot be disassembled.

PVC stand materials

To hang a standard-size (4-foot long) fluorescent or LED grow light fixture, the following materials build a stand that has one bar for hanging the light and two stabilizing feet. The light fixture should include a power cord, on/off switch, and means to suspend it from the frame (such as a metal chain and S-hooks).

• One 10-foot long, 1½-inch diameter PVC pipe
• Two 1½-inch slip (not threaded) T-shaped PVC pipe couplings
• Two 1½-inch 90º elbow PVC pipe couplings
• Four 1½-inch PVC pipe end caps

PVC stand construction steps

1. Use a ruler and marker to divide the 10-foot PVC pipe into 7 pieces: one 52 inches long; two 18 inches long; and four 8 inches long, which will leave no leftover pipe. Use a hacksaw to cut the pipe on the measurement markings.
2. Assemble the stand’s support legs:
   1. Push two 8-inch pipe pieces (as the stabilizing feet) and one of the 18-inch pipe pieces (the upright leg) into one of the “T” couplers.
   2. Repeat to build the other leg.
   3. Put the 4 pipe caps over each of the open ends of the leg feet.
3. Insert the top of each leg into an elbow coupler.
4. Insert the remaining long PVC piece into each elbow coupler.
5. Use metal chains wrapped around the stand’s top bar and S-hooks to suspend the light fixture.

Almost any clean container can be used for seed starting, provided it allows for good drainage and is at least 2 inches deep. Save money by reusing cottage cheese and yogurt containers, milk cartons, aluminum pans, and clear plastic food container clamshells.

You can buy standard (10-inch by 20-inch) plastic greenhouse trays (“flats”) with or without drainage holes. Flats without holes act like a saucer to contain drips from individual pots or flats with drain holes. Care must be taken to avoid overwatering undrained flats.

Seeds are typically sown in a tray of connected small pots (called cells): the connections make moving the plants around as a group easier, and the individual cells keep roots from tangling with neighboring plants as they grow. Cell packs (also called inserts or market packs) are square or rectangular and come in several configurations (frequently 4-packs or 6-packs); they are usually designed to fit into a standard flat. Cell packs are often made of flexible plastic, but in some cases can be biodegradable (such as peat or coconut coir fiber). Depending on the cell size, 10 to 120 plants can be grown per flat. Plastic cell packs and greenhouse flats can be reused for several years if handled carefully (flimsier plastic may tear).

Recommended individual cell sizes

• Lettuce, beets, spinach, kale, and other greens – 3 to 6 square inches
• Squash, cucumber, broccoli, and cabbage – 4 to 9 square inches
• Tomato, eggplant, and pepper –  9 to 16 square inches

Peat pots and other biodegradable seed-starting containers (dehydrated manure, coconut coir fiber, etc.) can be planted directly in the garden, but they may decompose slowly. Remove the pot base and any rim above the soil line, and cover the pot completely with garden soil. Removing the pot base helps roots establish without waiting for the pot to decay fully. Removing the rim prevents desiccation: if the edge of the pot sticks up out of the soil, it acts as a wick for soil moisture and dehydrates the transplant’s root zone.

Soil blocker tool

Some gardeners and small-scale commercial growers use soil blockers to produce transplants with less or no plastic. The tool takes moist potting mix and compresses and forms it into individual blocks that can be planted with seeds or seedlings.

Growing media for seed starting has three main functions: supply roots with nutrients, air, and water; allow for maximum root growth; and physically support the plant.

Growing media have many alternate names, often marketed as “potting soil,” “planting/potting/transplant mix,” and “container substrate.” Media should not be confused with true soil, which contains mineral components like sand, silt, or clay. Since container media do not contain soil (despite “potting soil” being a widely-used product label), they are categorized as “soilless.” Special “seed starter” media have a finer texture, which helps germinate very small seeds, but they are not essential for general vegetable seed starting.

Soil is too dense (approximately 75 pounds per cubic foot) to allow for good air and water movement around roots when used in a container. Soil also may contain weed seeds and plant pests or pathogens. Soils hold water very well in their small pore spaces, which can drown roots, especially in shallow containers like those used for starting seeds.

High-quality soilless media are lightweight (approximately 10 pounds per cubic foot) and fluffy (not easily compacted). They retain nutrients and hold water very well while having excellent drainage due to large pore spaces that allow excess water to drain easily. This high porosity promotes quick and extensive root growth. When fresh, they are also generally free of weeds, insects, and pathogens.

A potting mix combines several ingredients and is formulated to promote uniform plant growth. Common components include sphagnum peat moss, coconut coir, perlite, vermiculite, composted bark, compost, and rice hulls. Avoid heavy, dense potting mixes that contain “forest products.”

Soilless growing media often contains a small amount of fertilizer, enough to support plants for 6 to 8 weeks after germination. Potting mixes certified organic by the Organic Materials Review Institute (OMRI) differ from conventional products by using organic fertilizers and natural wetting agents like yucca extract. (Wetting agents help naturally water-repellent ingredients like peat moss to absorb moisture.)

Fill the containers

• Thoroughly moisten dry media before filling pots, especially if the media contains peat moss (most potting mixes do). Peat moss is hydrophobic when dry (it repels water), where water sprinkled on it will bead up and be more likely to roll off of the pot than soak in.
  • Place dry growing medium into a bucket or tub. Pour lukewarm tap water in slowly and work it into the growing medium by hand or with a tool like a scoop or hand cultivator. It should be as moist as a wrung-out sponge, not sopping wet.
• Fill the containers to within one inch of the top with the moistened media.
• Firm the media lightly with fingers or a block of wood to provide a uniform, flat surface.

Sow seeds

• How many seeds to sow in each tray or container depends on the crop. Transplants needing more space to grow, such as tomato, pepper, and eggplant, are often started singly (one seed per cell or pot) and later moved into larger individual containers 2 to 3 weeks after germination.
• Seedlings grown in rows are easier to label and handle at transplanting time than those started by broadcasting seeds randomly. You can make shallow rows for small-to-medium size seeds, ⅛ to ¼ inch deep, and 1 to 2 inches apart, using a rigid straight edge (such as a rigid ruler) or a finger.
• 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, uncovered or very lightly covered) with growing media and press down gently to ensure good contact between the seed and the media.
• A suitable planting depth is typically about twice the seed’s diameter. For example, a seed ⅛ inch wide can be planted ¼ inch deep.
• Cucumber, squash, melons, corn, and beans are grown for 2 to 4 weeks as transplants before planting in the garden. There’s no need to pot them up into larger containers. Sow two or three seeds per container or cell. After germination, keep the strongest individual in the cell and remove the others.

Temperature, humidity, and light

• Seeds need a warm, moist growing medium to germinate. Generally, a potting mix temperature of 65° to 75°F is good for germinating most vegetable seeds.
• Seed germination begins with water absorption, and an adequate and continuous supply of moisture is essential. A dry period will cause the death of the embryo once germination has begun.
• Spray some water from a mist bottle on the growing medium as needed to keep it moist.
• To increase humidity and temperature, cover the container with a piece of clear plastic (like commercially-sold seed-starting domes), or put the container into a plastic bag after sowing. The cover should not be in contact with the growing medium. Remove the cover as soon as sprouts appear.
• A seedling heat mat will speed up germination. Alternatively, drape clear plastic over a grow light fixture to create a warmth-trapping tent. The cover should rest on the frame holding the fixture and not on the fixture itself. Heat from the grow light’s fluorescent ballast or LEDs will be trapped inside the plastic tent and keep the temperature at 70º to 75º F.
• The top of a refrigerator is another warm location to hasten germination.
• After germination, seedlings can be given somewhat cooler conditions, with a 55 to 60°F night temperature and a 65 to 70°F day temperature.
• Keep fluorescent and LED light fixtures 2 to 6 inches above plants and lit for 14 to 16 hours a day using an outlet timer. Adjust as needed to keep plants stocky (not stretched and leggy). Tomato and cabbage family plants are prone to stretching toward the light.
• Attaching a thin chain or wire to either end of your light fixture will allow you to raise and lower the tubes to accommodate plant growth.

Water and fertilizer

• Water as needed to keep the growing media moist, but not wet. The goal is to keep the root system supplied with water and oxygen. (Too much water in the potting mix displaces oxygen.)
  • Wait until the top of the media is nearly dry before watering.
  • Top-down watering: apply water to the surface of the media and let it seep fully down into the pot and drain out of the bottom drain holes.
  • Bottom watering: allow the media to wick up water through the bottom drain holes by putting the containers into an undrained tray, pan, greenhouse flat, or other container with a shallow layer of water in the bottom.
• Common problems with seedling care are related to root moisture: a lack of uniform watering, overwatering, and letting the containers dry out.
  • Roots will stop growing and die due to a lack of oxygen when the growing media is saturated with water.
  • Fungus-like pathogens called “water molds” (Pythium, Phytophthora, and Rhizoctonia) cause root rot and damping-off disease that can quickly kill a container of seedlings if the growing media is kept too wet. If an infection occurs, dispose of the plants and growing medium, wash containers and tools to remove any debris, soak them for 1 to 2 minutes in a solution of 1 part chlorine bleach to 10 parts water, and rinse with clean water.
• Wait to fertilize: most potting mixes contain enough fertilizer to grow a seedling for at least 6 weeks without the need for more nutrients. When nutrients are becoming depleted and it’s time to supplement, use a water-soluble fertilizer and follow label dosage directions for indoor plants or transplants.
• Rinse off any liquid fertilizer that contacts foliage. Young seedlings are easily damaged by too much fertilizer, especially if they are under any stress from drying roots.

Potting-up seedlings to larger containers

If plants have not been seeded in individual containers, they must be potted-up (also called “pricking out” or “transplanting up”) into a larger individual container to give them proper growing space.

• One of the most common mistakes gardeners make is leaving seedlings in the seed flat too long. Crowded seedlings compete with each other for resources, and their intermingling roots are harder to disentangle for transplanting without significant damage.
• The ideal time to transplant young seedlings is when they are small and can quickly recover from transplant shock. This is shortly after the first “true leaves” appear above or between the cotyledons (the “seed leaves” that first appear during germination).
• Carefully dig up the small plants from the seedling flat or container using a tool like a butter knife, chopstick, or popsicle stick. Gently tease them apart in small groups to make it easier to separate individual plants.
• Handle small seedlings by their leaves, not their delicate stems.
• Make a hole in the growing medium into which the seedling will be planted. Set the seedlings so that the leaves and growing point are only slightly above the soil line (most of the stem can be buried).
• Firm the media around each plant and water gently.
• Continue to provide light for 14 to 16 hours per day, keeping the tops of the transplants 1 to 2 inches from the grow lights so they aren’t damaged.
• Gently brushing the tops of tomato transplants with your hand or a wooden stick will help control their height and increase stockiness. The overall dimensions of an ideal transplant is as wide as it is high (as opposed to tall and lanky/floppy).
• Sow tomatoes 5 to 6 weeks before you expect to plant them outside. You will end up with stocky 8- to 10-inch tall plants. If tomato transplants get very tall, you can dig a trench and lay the stems horizontally into it, leaving only 2 or 3 sets of leaves above ground. The buried portion of the stem will grow roots.

Winter sowing is a technique that allows you to get a jump on the growing season by growing transplants outdoors. Transplants for cool-season and warm-season crops are started outdoors in one-gallon milk jugs and other plastic containers and bags with adjustable openings for temperature and humidity control. The seedlings get light from the sun instead of grow lights. You’ll need to experiment with seeding times, container types, and location. Successful winter sowing requires close attention to the weather and needs of the seedlings.

Additional resources

Maryland Grows Blog topics:

• Seeds
• Transplants
• Peat-free Potting Mixes
• Winter Sowing: How I Get a Jump-Start on My Summer Flower Garden