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CIS 1209

Start Seeds Indoors with Success

Ariel Agenbroad, Stephen Love, Stuart Parkinson

Idaho green thumb how-to's


Starting vegetable, herb, and annual flower plants at home from seed is economical and rewarding. A greater variety of plant material is available by seed and the gardener skilled in seed starting enjoys more flexibility in planning and planting the garden.

Getting Started

Carefully schedule indoor planting to ensure your transplants are at the proper growth stage when the outdoor garden is ready. Begin by creating a planting schedule that includes crop varieties adapted to your climate and conditions. Information sources for crop and/or species lists include personal experience, neighbors and friends, Idaho Master Gardeners, and the local University of Idaho Extension educator.

Use the information in Table 1 to schedule transplant production based on the average last spring frost date in your locale. Cool-season crops, like broccoli, onions, and pansies, can be transplanted outside 2–5 weeks before the average date of last frost. Warm-season plants like tomatoes, peppers, and zinnias can be moved into the garden about 1–3 weeks after that date.

Table 1. Timing and temperatures for commonly transplanted vegetables, herbs, and annual flowers.
Vegetables and fruits
Broccoli, cauliflower 65°F–75°F 5–10 5–7
Cabbage, kale, and collards 65°F–75°F 5–10 5–7
Celery 65°F–75°F 10–18 8–10
Lettuces 65°F–70°F 7–10 3–5
Melons 80°F–85°F 5–10 3–4
Onions, leeks, and shallots 65°F–70°F 10–14 4–8
Peppers 75°F–85°F 10–14 6–9
Squash 70°F–85°F 7–14 3–4
Tomatoes 75°F–80°F 7–14 5–9
Basil 70°F–85°F 4–10 8–10
Parsley 65°F–70°F 10–25 6–8
Alyssum 55°F–70°F 8–14 8–9
Marigold 70°F–85°F 5–8 3
Nasturtium 65°F–70°F 10–14 3
Pansy 60°F–65°F 14–21 8–9
Snapdragon 60°F–85°F 7–14 8–10
Zinnia 70°F–85°F 7–14 3–4

Next, prepare your containers and planting medium for seed starting. To avoid disease, use new or clean pots and trays. Sterilize used containers by soaking them in a dilute bleach solution of one part bleach to nine parts water, then rinse and air dry.

Compressed peat pellets are convenient, but do not always provide enough rooting space for vigorous seedlings. It is easier to keep plants healthy in larger pots.

Use a moistened, lightweight, sterile potting mix, not garden soil. Do not reuse potting mix for seed starting and try to avoid mixes with large amounts of added fertilizer. You may add fine-textured compost or worm castings to your potting mix as a nominal source of nutrients.

Planting and Germination

Fill containers with potting mix to within ½ inch of the top. Plant seeds at the depth recommended on the packet. Label containers using permanent marker or graphite pencil and place containers inside a solid tray to catch water.

Although most crops grow well in consistent room-temperature conditions, those with ideal germination temperatures of 70°F or higher sprout much faster when supplied with bottom heat from a commercial seedling heat mat or other source. Remove the heat source once plants emerge.

Before plants emerge, irrigate with a fine misting nozzle that will not dislodge seeds or soil. Bottom-up watering also works well, accomplished by pouring room-temperature water into a tray holding the planting containers. If using this method, apply only as much water as the pots will completely absorb within a few minutes. Don’t let pots sit in standing water for long periods of time.

Caring for Transplants

Insufficient light is a major cause of transplant failure at home. To ensure stocky, sturdy seedlings, provide a source of artificial light even in sunny rooms or windowsills. Fluorescent tubes of any spectrum work; there is no need to purchase expensive plant grow lights. Hanging shoplight fixtures are commonly used. Suspend lights about 2–4 inches above the plants. Keep lights on for 12–16 hours per day and turn them off at night. A simple timer can schedule this automatically.

Proper watering is critical for producing healthy transplants. The trick is to consistently provide moderate conditions—not too wet, but never so dry that plants wilt.

Tomato seedlings
Tomato seedlings growing under ideal conditions for home transplant production. Photo by Ariel Agenbroad.

Add additional fertilizer only after the young plants have at least four leaves and then use a very dilute organic or conventional liquid fertilizer. Fertilization may not be necessary unless plants stop growing or turn yellow.

Using a small fan to blow air across the plants during the day will help prevent fungal problems and increase seedling toughness. Recognize that additional air movement increases water usage. Alternatively, brush your hand across the tops of the plants several times a day to help keep seedlings stocky.

Making the Transition Outdoors

Seedlings produced indoors are “soft” and will need to be gradually acclimated to outdoor temperatures, dry conditions, and intense sunlight even if the danger of frost has passed. This can be accomplished by moving the pots outside for progressively longer periods each successive day. This process of “hardening off” usually takes at least a week. Once the plants have adapted, plant them outside, but take any necessary steps to protect the newly established plants from sun, wind, and weather extremes.

About the Authors

Ariel Agenbroad—Area Extension Educator, Community Food Systems and Small Farms, University of Idaho Extension, Ada County

Stephen Love—Extension Consumer Horticulture Specialist, Aberdeen Research and Extension Center, University of Idaho

Stuart Parkinson—Extension Educator, University of Idaho Extension, Franklin County

Issued in furtherance of cooperative extension work in agriculture and home economics, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Barbara Petty, Director of University of Idaho Extension, University of Idaho, Moscow, Idaho 83844. The University of Idaho has a policy of nondiscrimination on the basis of race, color, religion, national origin, sex, sexual orientation, gender identity/expression, age, disability or status as a Vietnam-era veteran.

University of Idaho Extension


Physical Address:
E. J. Iddings Agricultural Science Laboratory, Room 10
606 S Rayburn St

Mailing Address:
875 Perimeter Drive MS 2332
Moscow, ID 83844-2332

Phone: 208-885-7982

Fax: 208-885-9046



Barbara Petty