Sowing Annual Seeds

In this guide, sowing annual seeds both indoors and outdoors is explored, as well the care and management of seeds.
Sowing Annual Seeds - Articles
Sowing Annual Seeds

Sowing seeds is a relatively inexpensive way to produce garden plants. In Pennsylvania, you can sow most annual seeds outdoors after the last frost date since the growing season is long enough for them to germinate, grow to maturity, and blossom before temperatures drop to near-freezing levels. If you want to have plants that bloom earlier, you can also sow seeds indoors in early spring and transplant outdoors after the last frost date.

Most annuals grow from seed. Seed companies carefully select seed that originates from plants with superior traits.

  • Standard seed refers to all seed used for sowing in annual flower beds and gardens. This seed is carefully selected and cleaned to remove any weed seeds or debris.
  • Dressed seed is dusted with a fungicide or a combination of insecticide and fungicide before packaging.
  • Pelleted seed is very small seed that is coated with a clay mixture for easier handling. This makes the seeds larger and more visible so they can be spaced properly with less need for thinning.

You can purchase packets of seeds from local stores and garden centers, through reputable catalogs, or directly from seed companies. If seed is left over after sowing, you can save it for the following sowing season if you store it in an air-tight container and keep it in a cool, dry place. However, even if you store them properly, as seeds age they can dry out and will be less likely to germinate. When sowing saved seed, you may need to double, triple, or quadruple the amount used.

You can also collect homegrown seed from your annual and biennial plants. Make sure the seeds are ripe (fully matured) when you collect them and allow them to thoroughly dry before you store them in a cool, airtight container. Many new annuals result from crosses between two parents with similar characteristics. Seeds saved from these plants often do not resemble the parents in color, size, or growth habit. The pollen that fertilized the flower may have come from inferior plants that were not cultivated or not preferred for growing in the garden. Sometimes, a plant self-seeds by dropping seed on the ground as a way of propagation. These “volunteer” seedlings may also develop into plants with these disadvantages. You may decide to let volunteer seedlings grow, thinning them where they germinate and transplanting them to another place, or treat them like weeds and pull them out.

Timing

Usually, gardeners set a planting date—an actual day on the calendar—to sow seeds or transplant seedlings and small plants into the garden. You can then plan seeding dates by counting back on the calendar the number of days to germination, plus a week or more for acclimation to allow seedlings to adjust to an outdoor environment. Consider scheduling separate planting dates for different varieties of plants if their days to germination vary and you want to transplant them into the garden at the same time. Some plants, such as dwarf marigolds, develop flowers in 5 weeks, while others, such as impatiens, take as long as 3 months or more. You can sow most summer annuals indoors 6 to 8 weeks before the last frost. Then they will be mature and ready to bloom outdoors on time.

You can best gauge specific times in the spring for sowing seed outdoors or transplanting by the average last frost date. There are three ranges of average last frost dates for Pennsylvania. Generally, in the southeast, or climatic zone 6, the last frost date ranges from March 30 to April 30. Regions close to Lake Erie, which modifies air temperatures, are also zone 6 regions. The biggest area of the state, which stretches from the northeast to southwest, has an average last frost date between May 1 and May 31 and is classified as zone 5. In the higher regions of the Allegheny National Forest, zone 4, the range of the last frost date begins on May 30 and ends June 30. Note that these dates are long-term averages and may vary a week either way, depending on local topography and environmental conditions.

Sowing Seeds in the Garden

Direct sowing requires a certain amount of planning and seedbed preparation. A well-made seedbed is necessary so that the seed will have enough air and moisture, and tiny roots will have a secure foothold. Seedbed preparation includes:

  • Choosing a sunny location for the garden.
  • Controlling any emerged weeds. Solarization is a method that involves placing plastic sheets over garden soil to raise soil temperature. The higher temperature encourages weeds to grow. You can then remove the weeds with a hoe or by hand.
  • Watering the soil and letting water settle until the soil is moist below the surface, but dry on top.
  • Raking the surface lightly until it is even and free of large soil clumps.
  • Thinly or sparingly scattering seeds.
  • Using the optimum spacing requirements listed on the back of seed packets to determine how closely seeds can be sown (see Table 1).

Table 1. Representative spacings for annual flowering plant seeds.

Scientific NameCommon NameSpacing (in inches)
Ageratum houstonianumfloss flower6–10
Althaea roseahollyhock12–24
Anterrhinum majussnapdragon8–18
Bellis perennistrue daisy, English daisy4–6
Calendula officialispot marigold10–12
Callistephus chinensisChina aster10–16
Campanula mediumCanterbury bells10–12
Catharanthus (or Vinca) roseusvinca, periwinkle8–12
Celosia argenteacrested and plume cockscomb8–12
Centaurea cyanuscornflower, bachelor button8–12
C. coronariumcrown daisy12–18
Cleome spinosaspider flower12–18
Consolida ambigua (Delphinium ajacis)larkspur12–18
Coreopsis spp.annual coreopsis8–12
Cosmos sp.cosmos10–14
Cynoglossum amabileChinese forget-me-not10–12
Dianthus barbatusannual sweet William8–10
D. chinensisChina pinks6–8
Digitalis purpureaannual foxglove12–24
Eschscholzia californicaCalifornia poppy6–8
Fuchsia hybridacommon fushsia12–18
Gaillardia pulchellaIndian blanket8–14
Gazania longiscapatreasure flower8–10
Gomphrena globosaglobe amaranth10–16
Gypsophila elegansbabysbreath10–12
Helianthus annuusgiant common sunflower12–24
Helichrysum bracteatumstrawflower8–10
H. peruvianumheliotrope10–12
Iberis amara coronariahyacinth-flowered candytuft8–10
I. umbellataglobe candytuft8–10
Impatiens spp.impatiens8–14
Ipomoea tricolormorning glory hybrids8–12
Lantana camaracommon lantana10–12
Lathyrus odoratus (bush type)sweet pea6–8
Limonium sinuatumstatice, sea lavender12–18
Lobelia erinusedging lobelia6–10
Lobularia maritimasweet alyssum8–10
Lupinus hyb.annual lupine18–24
Mathiola incana annuastock10–12
Mirabilis spp.four o’clock8–12
Myosotis sylvatica (M. oblongata)woodland forget-me-not8–10
Nicotiana alata grandifloraornamental tobacco8–10
Nierembergia caeruleacup flower8–10
Nigella damascenalove-in-a-mist8–10
Papaver spp.poppy6–12
Petunia x hybridapetunia8–10
Phaseolus coccineusscarlet runner bean8–10
Phlox drummondiiannual phlox6–8
Portulaca grandiflorarose moss6–8
Primula polyanthaprimrose6–8
Rudbeckia hirta gloriosagloriosa daisy10–20
Salvia spp.sage10–16
Tagetes erectaAfrican, American marigold12–16
T. patulaFrench marigold6–8
T. tenuifolia pumiladwarf marigold6–8
Tithonia rotundifoliaMexican sunflower12–14
Torenia fournieriwishbone flower6–8
Tropaeolum majuscommon nasturtium10–12
Verbena hortensis (V. hybrida)trailing verbena10–12
Viola tricolorJohnny-jump-up6–8
V. x wittrockianapansy6–8
Zinnia eleganszinnia6–12
Annual plants grown for characteristics other than flowers:
Scientific NameCommon NameSpacing (in inches)
Alternanthera amoenaJoseph’s coat8–10
Amaranthus caudatusamaranth, tassel flower16–24
Brassica oleracea capitataflowering cabbage18–24
Capsicum annuumornamental pepper8–10
Coleus blumeicoleus8–12
Hypoestes phyllostachyapolka-dot-plant8–12
Kochia scoparia tichophilabush, summer cypress14–24
Molucella laevisbells of Ireland8–10
Ocimum basilicumsweet basil12–18
Pennisetum selaceumpurple fountain grass30–36
Ricinus communiscastor bean30–36
Senecio cinerariadusty miller8–10

Spacing

Optimum spacing between plants is usually within the range of 6 to 12 inches. Depending on the circumstances, choose one number within the range to establish how closely to one another seeds can be sown. If you want a more compact appearance, choose a closer spacing range. Sowing seeds at the optimum spacing will help reduce damping-off, a fungus disease that can kill crowded seedlings. The less seedlings have to compete with one another during and after germination, the sturdier and bushier the plants will be. This simple step can prevent you from accidentally pouring a small pile of seeds into one section of the garden.

Correct spacing also encourages healthy growth by reducing the competition between plants for adequate sunlight, air, and soil moisture. In a seedbed, you can use grid lines to make rows for seed placement at optimum spacings. If a variety needs 14 inches between plants, mark grid lines 14 inches apart in both directions with the pointed end of a stick. Sow a few seeds at each of these intersections. Now you can remove seedlings that germinate away from intersections without destroying wanted seedlings. In early growth stages, it is difficult to identify plants by looking at seedlings, so gridline intersections also help to ensure that you do not remove desired seedlings. As an alternative to marking individual gridlines with the pointed end of a stick, you can create a series of gridlines by making several notches on a stick at recommended spacing intervals. Then, place the stick on the soil surface in both directions to create gridlines, and plant seeds at the appropriate notches.

Annual plants need room for growth. This enables them to be healthy at maturity and produce an abundance of flowers. So for both aesthetic and cultural reasons, be careful not to crowd plants.

Sowing

The correct sowing technique depends on the size of the seed (see Table 1). You can sow large seeds, such as nasturtiums, by hand. Lightly tap medium-sized seeds, like marigolds, from the seed packet. Since tiny seeds are difficult to handle by themselves, you can mix them with fine sand and distribute the mixture onto the soil. Alternatively, purchase a plastic handheld seed sower with a funnel-shaped spout that controls the distribution of seeds. Place seeds in this device’s reservoir, and control the flow of seeds though the funnel with the dial. Distribute seeds by lightly shaking the sower and depositing seeds at the desired intersections.

Sowing Depth

Environmental conditions, such as adequate soil moisture and warm temperatures, determine sowing depth. In early spring, when the soil is still cool and damp, seeds need only a light covering so that warmth from the sun can reach them. The usual depth for the average-sized seed in the spring is 2 to 3 inches. During later spring and summer, sow seeds deeper to prevent the heat of the sun from drying them out too quickly. The usual depth for an average-sized seed later in the season is 2 to 3 inches deep. After making sure that the furrows are the proper depth, check the soil moisture. If the soil is dry, gently water the furrows before sowing. You can check soil moisture in a few different ways:

  • Visual observations: Does the soil look dark as if it has been watered recently, or does it look pale and dried out? Are there any visible signs of moisture underneath the surface?
  • Touch: Do you feel any moisture when you put your fingers into the soil at the depth that seeds were sown?
  • Water sensors: You can purchase water sensors at most garden supply stores. The absence or presence of water activates them, and indicators, such as a color change, alert you when water needs to be added.

After the seeds are sown, tamp or firm the soil over the seeds to ensure that they have good contact with the soil.

Labeling the Seedbed

Properly label each planting to avoid possible confusion after preparing seedbeds and sowing seeds. Use water-proof markers to create labels on wooden or plastic stakes to ensure that labeling is visible at all times. Since many young seedlings are difficult to tell apart and you may want to transplant some of them to another location in the garden at a later time, this effort will pay off.

Watering

Maintain a constant moisture in the seedbed until seeds have germinated. Use a very fine spray or mist to gently water soil without washing out the seed. Do this daily except on bright sunny days when the seedbed may need light waterings two or three times. As seeds germinate, give young seedlings more generous waterings less frequently.

If you cannot water lightly each day, or if garden locations are exceptionally sunny, there are a few techniques that can slow evaporation, or loss of water from the soil. Try covering rows with loose floating row covers. The covers are made with polypropylene materials and you can either place them directly over the seeds or over hoops stuck in the ground so that the covering does not crush emerging seedlings. Alternatively, lay a piece of burlap or loose-mesh cloth over the seedbed and secure it with stakes or rocks. Covers serve a dual function by slowing evaporation and preventing rain from washing seeds away. They also help soil stay in contact with seeds, which is vital for germination.

Some soils tend to crust on the surface, which creates a barrier for emerging seedlings. To prevent crusting, create a furrow in the soil and fill it with moistened compost. Then sow seed into the compost and cover it with another layer. Water the newly planted seeds thoroughly, using a light mist. Floating row covers or misted newspaper covering the top of the soil can also reduce crusting and prevent the top layer of soil or compost from drying out.

Whether you use covers to prevent wash-out, slow evaporation, or prevent crusting, remove them promptly as soon as germination begins; otherwise, plants will stretch due to lack of sunlight. If you remove them too late, there is a chance that you might pull some seedlings from the ground when you finally do take the coverings off. Other materials, such as plastic covers, may trap heat underneath, which can kill seedlings.

Sowing Seeds Indoors

Annuals can benefit from warmer indoor temperatures during germination and early seedling growth before you transplant them outdoors when temperatures are more favorable. It is a good idea to sow seeds for annuals that germinate and grow slowly indoors, in the early spring. When deciding whether or not to take on the challenge of starting seeds indoors, weigh the disadvantages against the advantages.

Advantages

  • Easier if a large number of plants are needed.
  • Cheaper than purchasing individual plants.
  • Local garden centers have a greater selection of plants available as seed than as transplants.
  • Plants can be ready to transplant and will flower earlier.
  • When sowing multiple species, germination dates can be staggered so that all plants will flower within the same time frame.
  • Greater flexibility of staggering plantings for a specific crop.

Disadvantages

  • Resulting seedlings are sometimes weak from lack of light and may do poorly when placed in the garden.
  • Increased disease susceptibility.
  • Not all seeds germinate and not all germinate at the same time.
  • Seeds have varying germination requirements that should be understood.
  • Can be time-consuming, and diligent care is required.

You can use containers such as flower pots, seed trays, flats or pans, peat pots, peat pellets, and planting blocks for germinating seeds (see Figure 1). Of these container options, you can only place peat pots, pellets, and planting blocks directly in the ground when it is time for transplanting. Seed flats are more practical for sowing many seeds of one variety. It is simpler to have one container for each variety of seed so that all seeds in the container germinate at the same time. Wood and plastic are the most common materials used to make flats. Seed germination flats commonly range in size from 2 to 3 inches deep, 12 to 16 inches wide, and 18 to 24 inches long. You can make your own seed flats from scraps of wood with drainage holes drilled in the bottom or ½-inch spaces left between bottom slats. You can also purchase seed starting kits, which include crates and plastic lids, from a local garden center. When transplanting the seedlings, separate them by slicing through the soil between plants with a knife, and transplant each piece of soil.

Pots are usually made out of plastic or clay. If you choose a clay pot, soak it in water overnight, since dry clay tends to pull moisture away from the seed-starting mix. Plastic pots do not dry out as rapidly as clay pots, so be careful not to overwater them. The best size range for a pot is from 4 to 6 inches in diameter.

Whatever container you choose, add enough seed-starting to fill it 3/4 of the way full. Then level the mix, thoroughly soak it with water, and tap the container on a surface to firm the soil. Next, lightly press tiny furrows into the mix. This marks a uniform depth and distance for the seeds. The proper depth and distance between seeds depends on the seed size. For example, a furrow for small seeds may be 2 inches apart and somewhat shallow, while rows of large seeds would be 3 inches apart and a little deeper. To be sure, check cultural suggestions on the back of most seed packets before making furrows and sowing seeds. You should not sow most seeds deeper than two or three times their diameter, which is generally about 3 inches deep.

You can also fill the furrow with vermiculite so that seedling roots are easier to remove when transplanting. You may prefer to top the seedbed with an additional ¼ inch of vermiculite to prevent the surface from crusting and to enable seedlings to emerge easily. Certain seed-starting mixes contain materials that are very fine in texture, allowing seedlings to be transplanted easily.

Alternatively, you can sow seeds directly into a container with individual cells, or sections, such as a market pack or cell pack that may consist of 16 to 48 cells per flat. Sizes range from 1½- to 2½-inches deep and ¾ of an inch to 2 inches square. This container design will eliminate the need for transplanting seedlings into individual containers after they germinate. Instead, sow a few seeds in each cell, and after germination, thin seedlings so that only one seedling grows in each cell. If the goal is to reduce the amount of transplanting, select larger cell sizes. Seeds sown in smaller cells will need to be transplanted sooner than seeds sown in larger ones.

Paper pots are another option. These containers are constructed from newspaper molded into a pot shape. Paper pots, however, dry out readily, and fungus may grow on the moist paper and spread from the seed-starting mix to the seedling.

A peat pellet consists of a flat disk of compressed peat moss infused with fertilizer and held together by plastic mesh. When moistened, the disk expands and is used as a pot. Be careful to cut the mesh netting away from the top of the peat pellet prior to sowing seed. When transplanted to the garden, roots grow through the medium and into the soil more easily than with the peat pot.

Soil-block makers mold soil mix into small planting blocks. Then place seeds on top of the small blocks on cubes, and, as with peat pots, roots grow through the medium.

After sowing seeds in a container, consider the proper amounts of water, warmth, shade, and humidity that will be needed for optimum germination and growth. Balance these factors with one another. If the tray is at the proper temperature but is not well drained, some seeds may germinate but never emerge because they have rotted in the medium. Likewise, seedlings will rot before germinating in a tray that is both damp and cool.

Water

After sowing, use a water bottle with a fine mist to moisten the seed starting mix without disturbing seeds. Seeds can then get moisture without washing out of the mix. Moisten the seed-starting mix in a larger container by standing the container in a pan of water until moisture rises to dampen the surface. Do not place the container in water that is too deep or it will run over the top of the container and completely soak the seed starting mix. If this happens, lift the container out of the water and drain it.

Temperature

Successful germination requires warm temperatures. Generally, the optimum temperature for germination is between 65°F and 75°F. Though it may be difficult with some residential heating systems, keep the area around the container at a constant temperature within this range. If the temperature decreases to below 65oF, place the container close to a heat source, such as a radiator. However, take care when putting containers close to a heat source. If you place them too close, plants will desiccate and the soil will dry out too rapidly. As one approach to maintaining an even and warm heating system, use seed germination mats with a temperature-controlling thermostat to maintain a uniform temperature. Do not use an unprotected heating pad, since there may be a risk of shock or fire.

Light

Monitor the amount of light reaching containers to prevent them from drying out too quickly. Use a tent made from a sheet of brown paper to reduce the container’s exposure to light. Never allow sunlight to shine directly on plastic-covered containers. Be sure excessive heat does not get trapped under the tent, elevating temperatures and injuring newly emerged seedlings. Consult seed packets for light level requirements that are specific for the seeds being sown, as some seed needs certain light levels to germinate.

Germination requires partial shade, but most seedlings need bright light for strong growth. Seedlings prefer natural light, so the best place to set the flat of seedlings is on a windowsill. Even though it increases light intensity, refrain from putting containers in direct light until the plants have adjusted to their growing environment. Instead, place the flats in indirect light. Turn the containers regularly so that each side is exposed to the sunlight. This will help to avoid lopsided growth from exposure to a side, or angled, light source. If you use windowsills as light sources, the number of brightly lit windowsills in the house limits the number of plants grown in the house. A lack of adequate humidity and an increased mid-day temperature that is often too warm for seedlings are other disadvantages to using windowsills.

If no windowsills are available, place seedlings under artificial lights, such as fluorescent lamps. Certain lamps are made especially for growing plants and seedlings, but they may not be necessary and can be expensive. It is more economical to buy fluorescent lights labeled
as white, warm white, daylight, or cool white, which are often sold as shop lights. They are not as good as sunlight, but they provide better quality light than incandescent light bulbs without producing as much heat. You can purchase fluorescent light stands or make your own structures for supporting fluorescent light fixtures (see Figures 2 and 3). Most stands have waterproof trays to hold containers and light fixtures for fluorescent light tubes that can be raised or lowered according to plant height.

You can plug lights into a timer so that day-lengths are uniform. The length of time that lights are turned on depends upon the variety of annual plant being grown. Only place annuals together under one light fixture if both plants have the same light requirements. Most annuals need 18 to 20 hours of light per day. If exposed to less light than this, plants may develop weak stems. Some plants only require 10 to 12 hours of light. These include calliopsis, China aster, cornflower, gaillardia, petunia, phlox, poppy, rudbeckia, salpiglossis, scabiosa, snapdragon, and verbena. To be sure about the light requirements of individual varieties, read the cultural suggestions on the back of seed packets.

Figure 2. If no windowsills are available, place seedlings under artificial lights, such as fluorescent lamps.

Figure 3. You can purchase light stands or make your own structures to support fluorescent light fixtures.

Humidity

Increased levels of humidity around the seeds can improve germination. A clear plastic bag, a single layer of plastic wrapped around a container, or a plastic seed tray lid can increase humidity. If using a plastic bag, allow some rounded air space above the tray before the bag is closed. If using a pot, tuck the open end of the plastic bag underneath the pot, or secure a rubber band around the pot to hold the bag in place. Be sure to leave sufficient air space over the top of the container so that seedlings are not restricted when they germinate. Insert plant labels or stakes into the seed-starting mix to keep the bag or plastic wrap from sagging. Inside, condensation can run off the rounded sides and top of the bag. As soon as the seedlings emerge, remove the bag.

A glass pane can also preserve humidity. Lay a sheet of glass over the tray or pot with brown paper on top for shade. Wipe condensation off of the glass daily. As when using a plastic bag or wrap, once seedlings begin to germinate, remove the glass pane. You will need to lightly water the seedlings when the surface of the seed-starting mix begins to dry. Water the mix to levels similar to germinating conditions.

Cold Frames and Hotbeds

While you may not have access to greenhouses, you can build a cold frame or hotbed. Both are box-like structures made of boards or cement with a front about 1 to 2 feet deep and a back section of about 2 to 3 feet deep. They are approximately 6 feet long and 3 feet wide with a slanting glass sash on the top that preferably faces south to take advantage of the most light available. By trapping solar heat, cold frames and hotbeds protect plants, cuttings, and seedlings from intolerable cold temperatures. The trapped heat also keeps the interior of the cold frame/hotbed warm during the night. Hotbeds have an advantage over cold frames because they use electric heating units to raise the temperature above the level of the solar heat produced by the sun. To prevent temperatures from becoming dangerously hot in the late spring or summer, prop open and shade sashes. Then you can place seedlings in the garden after the last frost date.

Thinning

Most annual seeds take 1 or 2 weeks to germinate. During this time, two seed leaves, or embryonic leaves, grow from each seed. These are not true leaves, but part of the sprouting seed that appears more swollen and rounded than mature leaves. A few days later, true leaves, which look like typical plant leaves but smaller, will develop above the seed leaves.

At this time, you can thin the seedlings. Remove undesired seedlings so that the remaining, strongest seedlings are spaced 2 inches apart. Be careful during this process not to damage chosen seedlings as they are very delicate at this stage. First, check the soil for adequate moisture. If it is not already fairly moist, water it so you can remove seedlings without disturbing the roots of the plant that is left to mature. If seeds were evenly sown in parallel furrows, thin twice with a ten-day interval between the two thinning dates. You can also thin seedlings by clipping off the unwanted seedling right above the sowing medium. Alternatively, you may want to save as many seedlings as possible and transplant them to another site. If you handle them with care and keep roots intact, you can transplant the seedlings.

Suggested Further Reading

Armitage, A. 2001. Armitage’s manual of annuals, biennials, and half-hardy perennials. Timber Press. Portland, OR.

Damrosch, B. 1988. The garden primer. Workman Publishing Company, Inc. New York, NY.

Hessayon, D.G. 1996. The bedding plant expert. Sterling Publications. New York, NY.

Holcomb, E.J. (Editor). 1994. Bedding plants IV. Pennsylvania Flower Growers, Inc. Ball Publishing. Batavia, IL.

McKinley, M. (Editor). 1999. Ortho’s all about annuals. Meredith Books, Des Moines, IA.

Nelson, P.V. 1998. Greenhouse operation and management, fifth edition. Ball Publishing. Batavia, IL.

Prepared by Kathleen M. Kelley, associate professor of consumer horticulture, James C. Sellmer, associate professor of ornamental horticulture, and Phyllis Lamont, consumer horticulture center library coordinator.

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