Growing Annual Plants from Seed

Introduction 

Growing annual flowers and vegetables from seed can be rewarding and fun. By beginning with seeds rather than purchased transplants, gardeners can 1) increase the variety of plants they grow (since more varieties tend to be available in seed form), 2) better control growing conditions and inputs like pesticides, and 3) grow large numbers of plants relatively inexpensively. 

Seeds for annual flowers and vegetables can be directly planted in garden beds or outdoor containers, sown in cold frames outdoors, or started indoors for transplanting when weather warms. 

Use good, viable seed. Seed collected from last year’s garden may not produce predictable results because of the interbreeding of varieties. Old seed, unless carefully stored in a cool, dry location, often germinates poorly. It is usually more satisfactory to buy fresh, new seed when growing garden annuals and vegetables. Most seed packets are dated using phrases such as ‘Packed for (Year).’

Garden Preparation for Annual Plants

Sowing seed directly in the garden is the simplest way to start plants. It is usually safe to sow annual seeds outdoors when trees are beginning to produce leaves. See Table 1 for when to start seeds of specific plants. 

Before seeding or planting annual flowers or vegetables in new beds, spade the garden area 6 to 10 inches deep. If indicated by a soil test, add organic matter to improve soil structure. Soil organic matter helps keep the soil from becoming too compact and helps build healthy soil structure. See Fact Sheet 7.235 “Choosing a Soil Amendment” for more information. 

In established beds, organic matter may not be needed, and soil should be loosened using as light a hand as possible. Broadforks or spading forks can lift and aerate soil without damaging soil structure; be careful not to overwork soil and do not work in soil that is wet. Both over-tilling and working wet soils damage soil structure and lead to soil compaction, which limits plant performance. 

When ready to plant seeds, rake the surface smooth and remove or break down large clods. Plant seeds in rows or blocks and cover with fine soil to the depth indicated on the seed packet. Mark the seeded areas with identifying labels. 

Keep the seedbed moist at all times. When seedlings appear, thin plants to half the height they are supposed to attain. For example, if the mature height of the plant is 12 inches, thin the plants to be six (6) inches apart. For tall, spike-like annuals such as snapdragons, larkspur, and foxglove, thin to one-fourth of their mature height for a fuller, more showy effect.

Cold Frames

For an early start, sow seeds in a cold frame (Figure 1). Seed may be started as much as six weeks earlier than outdoors. Seeds can be planted in a bed covered by a cold frame or grown in pots to be transplanted to another part of the garden. 

Locate the cold frame on the south side of a garage or dwelling. If built with a tight-fitting lid, the cold frame will hold sufficient heat from the sun to keep seed and seedlings warm at night. On warm, sunny days (50F or warmer), prop the lid open to prevent the buildup of excessive heat. Close the lid in the late afternoon to trap enough heat for cold evenings.

As the season progresses, gradually expose the plants to longer periods of outside temperatures, as long as the air temperature does not go below 50F. Treated in this way, seedlings develop into sturdier plants that are better able to adapt to fully exposed garden conditions at transplant time. This is particularly true of the hardy annuals and biennials that prefer to develop in cooler temperatures, e.g., pansy, ageratum, lobelia, verbena, cabbage, broccoli, and lettuce. Use Table 1 to determine when to start seed in the cold frame. 

Table 1: Starting times for seeds grown indoors and in cold frames, shown as number of weeks to start seed before average frost-free date. 

Plant name	In cold frame	Indoors
Ageratum	6	8
*Amaranth	4	6
*Bachelor’s button	4	4
Broccoli	6	4a
Cabbage	6	4a
*Calendula	4	4
*California poppy	4	4
Calliopsis	4	6
Cauliflower	6	4a
China aster	4	6
*Cosmos	4	4
Dahlia	6	8
Dimorphotheca (African daisy)	4	4
*Gaillardia	4	4
*Gomphrena	4	4
Larkspur	4	6
Lettuce (head and semihead)	6	4b
Lobelia	6	8
*Marigold	4b	4
*Morning glory	4	4
*Nasturtium	4b	4
Pepper	4b	6
Petunia	6	8
Phlox (annual)	4	6
*Poppy (Shirley)	4	4
Salvia	4	6
*Scabiosa	4	4
Snapdragon	6	8
Statice	4	6
*Straw flower	4	4
*Sweet alyssum (Lobularia)	4	6
Tomato	4b	6
Verbena	6	8
Vinca (annual)	4	6
*Zinnia	4	4
*Plants best suited for direct seeding in garden two weeks prior to the average frost-free date.  aShould be kept in coolest room. Best at 55F to 60F.  bIf outside temperatures are below 20F at night, delay planting or use artificial heat to keep temperatures above 50F.

Starting Seed Indoors

Start seeds indoors four to eight weeks before the average frost-free date in your area. Starting too early usually results in spindly plants due to crowding and lack of sufficient light. 

Almost any container with drainage holes in the bottom will work for planting. Paper milk cartons cut in half, Styrofoam cups, tin cans, plastic trays and pots are common containers used. For convenience, however, you may wish to start plants in the plastic trays and pots available at garden supply centers. 

Use soilless potting mix or potting soil. Any commercial potting mixes usually are suitable and do not have weed seeds. They are, however, more expensive than soil mixes you can make at home. If you use soil from the yard, it should be topsoil that is well drained and blended with organic matter. The best soils are often found around established shrubs and trees. Add sphagnum peat and sharp sand to the soil in a ratio of about one-third volume of each and mix thoroughly. 

To kill weed seeds and pathogenic soil fungi, place the soil mix in shallow trays or baking pans in an oven for 45 minutes at 250F. For the best results, the soil should be moist. Be advised that this process can generate unpleasant smells in the home. 

Fill containers firmly but do not pack. Allow ample space from the soil/potting mix surface to the rim of the container. Place seeds on the soil surface. Carefully cover the seeds to the depth indicated on the seed packet. A piece of window screen or old flour sifter will help to apply fine soil evenly. Do not cover seeds too deeply, as this may reduce or prevent germination. Generally, cover no more than four times the seed’s diameter. 

If you use compartmentalized trays or individual peat pots, place two or three seeds in each pot.  

Apply a fine spray of water to avoid washing out the seed or causing them to float to the soil surface. Household spray bottles are suitable. Cover the containers with plastic sheets, humidity domes, or panes of glass. Most seeds germinate best at warmer soil temperatures; consider using a heat mat to speed up germination. When seeds germinate, remove the humidity coverings and place them in bright light. For cool-season plants like those listed above, remove the heat mat and place in a cool room.  

Even in a sunny window, supplemental lighting may be required to produce vigorous plants. When the seedlings have developed the first true leaves (the leaves above the cotyledons or ‘seed leaves’), thin to one plant per container if using partitioned trays or peat pots. Use tweezers to pinch off unwanted seedlings rather than pulling them to avoid disturbing the remaining seedling. 

If multiple seeds were planted in larger containers, transplant seedlings into individual peat pots or other small containers. An alternative is to thin the seedlings, so they are spread about 1 1/2 to 2 inches apart and leave them in the larger containers.  

Water seedlings carefully. Small containers used for starting plants dry out quickly. On the other hand, soil kept soaking wet inhibits seedling growth and may kill the plants. 

About one week before planting-out time, gradually expose seedlings to longer periods outdoors (unless temperatures are below 50F) to harden them. At the same time, reduce watering to a minimum as long as plants do not wilt. This will help the plants adjust to full exposure without undergoing undue shock at planting time.