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When to Start Seeds Indoors

By: Sherri Ribbey
This handy guide shows you the right time and temperature for starting seeds indoors and the best time for transplanting them outdoors so plants will thrive.

Start seeds at the right time

You have the seed starting mix, growing trays and ordered packets of seed — now are you wondering exactly when to start seeds indoors? Getting the timing right can make the difference between plants that lanquish and those that thrive. Planted too early or too late, seeds can rot or young plants may struggle to get established.

Start by finding the last expected frost date for your area. From there count back the appropriate number of days or weeks and you have the planting date. The charts below provide the timing as well as the germination temperature for starting 24 common flower and vegetable seeds. In addition, you'll find the recommended outdoor temperatures that are best for transplanting. Though the germination temperature needs vary, once seeds have sprouted, most seedlings do just fine in a sunny room that’s 60 to 70 degrees F until it’s time to move them outdoors.

Harden off plants to acclimate to the outdoors

Instead of taking your tender seedlings from the warmth and shelter of your home into the bright sunlight and gusty winds of spring be sure to give them some time to acclimate to their new growing conditions. Harden off young plants by setting them in a sheltered spot outside out of direct sunlight for a couple of hours to start with each day. Gradually increase the time they spend outdoors over the course of a few days. If there's a late-season storm skip that day and resume when the weather is warm again. Get more details and tips in our article How To Harden Off New or Overwintered Plants.

Seed-starting jargon

Before getting started here’s a quick guide to some of the technical terms used in the charts below.

Germination

When a seed produces growth it has germinated.

Needs light to germinate

Light is an important element that helps some seeds germinate. There’s no need to make a hole when you’re planting these seeds. Just sprinkle them on the surface of the seed-starting mix and press them lightly into the mix to ensure they make contact and stay in place.

Seed germination temperatures

Though it's not strictly necessary, this is the temperature range to keep the potting mix (use a seed starting heat mat) to encourage the quickest germination.

Stratification

Some seeds need a period of cold and moisture to break down a heavy seed coat in order to germinate. Gardeners can mimic this natural process by placing seeds in a container of moist sand or vermiculite and storing it in the refrigerator at 41 degrees F for at least 4 weeks. Perennials seeds often need stratification.

Scarification

Tough seed coats of sweet peas, nasturtiums and others need to nicked, cracked or softened so water can penetrate and encourage the seed to grow. Use a file or sandpaper to make a shallow nick or cut in the seed coating. You can also soak the seeds in water overnight to soften the coating but no longer or they may rot.

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Seedlings that can be transplanted outside in early spring

These plants can can be transplanted outdoors in spring when it’s 40 to 50 degrees F during the day.

Plant name Botanical name Seed germination temperatures Days to germination Weeks to planting seedlings outdoors Comments
Broccoli
Brassica oleracea italica
70 to 75°F 7 to 10 5 to 7 Set outside up to 2 weeks before last frost
Cabbage
Brassica oleracea capitata
70 to 75°F 5 to 10 5 to 7 Can take a light frost
Cauliflower
Brassica oleracea botrytis
75 to 80°F 5 to 10 5 to 7 Prefers cool temps or won’t produce head
Pansy
Viola x wittrockiana
65 to 75°F 7 to 15 8 to 12 Benefits from stratification; can take frost; foliage freezes at 10°F
Sweet peas
Lathyrus odorata
55 to 65°F 14 to 21 4 to 6 Benefits from scarification; can also be direct-sown outdoors; can take a light frost

Seedlings that can be transplanted outdoors after average last frost date

When temperatures are starting to warm up and the last average frost date for your area has gone by you can transplant these young plants out in the garden.

Plant name Botanical name Germination temperatures Days to germination Weeks to planting outdoors Comments
Artichoke
Cynara scolymus
70 to 80°F 18 to 21 6 to 8 Benefits from stratification; takes 5 to 7 months from sowing to crop
Bells of Ireland
Moluccella laevis
65 to 68°F 12 to 21 6 to 8 Needs light to germinate; benefits from stratification; doesn’t like roots disturbed — use peat pots
Butterfly weed
Asclepias tuberosa
70 to 75°F 21 to 28 6 to 8 Doesn’t like roots disturbed — use peat pots
Celery
Apium graveolens
70 to 75°F 20 to 25 10 to 12 Seedlings need constant moisture to develop
Foxglove
Digitalis purpurea
65 to 70°F 15 to 20 10 to 12 Needs light to germinate; biennial so may not bloom first year
Hyacinth bean
Lablab purpureus
65 to 70°F 5 to 12 6 to 8 Benefits from stratification; can take up to 80 days to get beans
Sage
Salvia officinalis
60 to 70°F 7 to 21 6 to 8 Needs light to germinate; let dry out between watering to avoid rot
Statice
Limonium sinuatum
68 to 75°F 12 to 20 8 to 10 Needs light to germinate; plant seedlings outdoors 12 in. apart
Sweet alyssum
Lobularia maritima
65 to 70°F 8 to 10 4 to 5 Plant seedlings outdoors 6 to 8 in. apart

Seedlings you should wait to transplant outside until the danger of frost has past

These warm-weather lovers are the last seedlings to be transplanted outdoors. They do best after all danger of frost has gone by and the soil has warmed — usually when daytime temperatures are around 70 degrees F and the nights are in the 50s it's safe to transplant this group.

Plant name Botanical name Germination temperatures Days to germination Weeks to planting outdoors Comments
Celosia
Celosia spp. and hybrids
70 to 80°F 8 to 14 4 to 6 Pinch tip when 8 to 12 in. tall for branching
Coleus
Plectranthus spp. and hybrids
70 to 75°F 10 to 14 6 to 8 Needs light to germinate; pinch tip when plants have four to six sets of leaves
Cucumber
Cucumis sativus
68 to 72°F 7 to 10 4 to 6 Don’t disturb roots when transplanting
Cup & saucer vine
Cobaea scandens
70 to 75°F 10 to 30 6 to 8 Benefits from scarification; vigorous grower, plant near a strong trellis
Eggplant
Solanum melongena
80 to 90°F 10 to 14 8 to 10 Don’t set out too early, cool temps stunt growth
Floss flower
Ageratum houstonianum
78 to 82°F 5 to 10 6 to 8 Needs light to germinate; plant seedlings outdoors 9 to 12 in. apart
Joseph's coat
Amaranthus tricolor
70 to 75°F 10 to 15 3 to 4 Let dry out between watering to avoid rot
Nasturtium
Tropaeolum majus
60 to 65°F 7 to 14 4 to 6 Benefits from scarification; can also be direct-sown outdoors; no need to fertilize after planting outside
Pepper
Capsicum annuum
80 to 90°F 7 to 10 6 to 8 Don’t set out too early; cool temps stunt growth
Tomato
Solanum lycopersicum
80 to 90°F 7 to 14 5 to 6 Plant leggy seedlings deep and they’ll form roots all along the stem for added support

You Might Also Like:
How to Harvest and Plant Milkweed Seeds
Perennial Seeds to Sow in Fall
How to Save Tomato Seeds
How to Collect Seeds from Your Garden

Published: Oct. 17, 2019
Updated: Jan. 20, 2021
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