Starting seeds indoors
Raising seedlings indoors comes down to a handful of choices: the container, the mix, warmth, light and water. A soilless seed-starting mix is the right base, not garden soil. Germination is ideal at a soil temperature of 65 to 75 degrees F. A bright window is usually inadequate by itself, so supplemental grow lights are needed. Most annual seeds take 1 to 2 weeks to germinate. This guide follows your seeds from the order through hardening off and transplanting, with timing counted from your average last frost date.
When: Late winter and spring (Jan–Apr)
When to do it
Work backward from your average last frost date. Eggplant and pepper seeds are started indoors about 8 weeks before that date. Watermelon seeds go in about 6 weeks before it. Cucumber, muskmelon and squash seeds should be started no more than 4 weeks before it. Tomatoes can be sown indoors or in a greenhouse 6 to 8 weeks ahead of moving the seedlings outside. Summer annual flowers can be sown indoors 6 to 8 weeks before the last frost. Popular warm season vegetables are sown 4 to 8 weeks before they go outdoors. Planting too early can expose tender tissue to late frosts, or let seeds rot in cold soil before they germinate. Planting too late shortens the growing season and may keep warm-season crops from setting and ripening fruit.
What you need
Every seed-starting container needs drainage and a depth of at least 2 inches. A seed-starting flat is about 2 to 3 inches deep, 12 to 16 inches wide, and 18 to 24 inches long. A cell-pack flat holds 16 to 48 cells, each about 1.5 to 2.5 inches deep and 0.75 to 2 inches square. A good seed-starting pot measures about 4 to 6 inches in diameter. For the mix, choose a soilless blend of sphagnum peat moss, perlite, and vermiculite. A homemade version uses one-third pasteurized soil or compost, one-third sand, vermiculite or perlite, and one-third coconut coir or peat moss. To pasteurize soil, heat it in a 250-degree oven until the soil itself reaches 180 degrees, then hold that temperature for 30 minutes. You also need grow lights, and blue light or mixed-spectrum bulbs are suitable for starting seeds and leafy greens. Keep leftover seed in a tightly sealed jar in the refrigerator at 32 to 50 degrees F.
Step by step
- Step 1
Moisten the seed-starting mix before you sow
It should feel damp like a sponge, not soggy, at planting time.
- Step 2
Sow each seed at the right depth
Set it about two times the seed's own width deep in the mix.
- Step 3
Plant two or three seeds per cell as insurance
Once they sprout, keep only the strongest seedling and thin out the rest.
- Step 4
Give each seedling its own container instead of a shared flat
Roots of neighboring seedlings in the same flat tangle together and get injured when you separate them for transplanting.
- Step 5
Thin seedlings to about two inches (5 cm) apart once they are up
If you sowed in parallel furrows, thin them twice, with about a ten-day gap between the two thinnings.
- Step 6
Keep the mix consistently moist through germination
If it dries out even once during this stage, the seed is no longer viable.
- Step 7
Hold the mix at 65 to 75°F (18 to 24°C) for best germination
Keep air temperature above 60°F (16°C), and add bottom heat if needed, since the mix can run 5°F (3°C) cooler than the air.
- Step 8
Cover the container with clear plastic or a plastic bag to hold in humidity
Remove that cover as soon as the seedlings emerge.
- Step 9
Add supplemental grow lights once seedlings are up
Window light alone is usually too weak, leaving seedlings leggy.
- Step 10
Position the lights two to four inches (5-10 cm) above the seedlings at first
Raise the lights as the seedlings grow, moving to four to six inches (10-15 cm) above their tops once they are established.
- Step 11
Run the lights twelve to sixteen hours a day
Between natural and artificial light, transplants need sixteen to eighteen hours total, though most seedlings need eighteen to twenty hours and some need only ten to twelve.
- Step 12
Check that the light reaching seedlings is strong enough
Ambient light below about 50 to 150 micromoles per square meter per second (50-250 foot-candles) is not enough alone; use a blue or mixed-spectrum bulb.
- Step 13
Start fertilizing once the first true leaves appear
Feed at every other watering with a water-soluble starter fertilizer, and use plain water the rest of the time.
- Step 14
Alternatively, wait until the seedlings have several sets of true leaves before feeding at all
From that point, fertilize about once a week with a fertilizer mixed at one-quarter strength.
- Step 15
Water from the bottom by setting the tray in water
Let it sit and absorb water for three to ten minutes.
- Step 16
Watch for signs of overwatering, especially algae on the soil surface
Seedlings need frequent, shallow watering while they are small, while mature plants need deeper, less frequent watering.
- Step 17
Set up a small fan once seedlings have two true leaves
A gentle, steady sway from the fan helps strengthen their stems.
- Step 18
Skip miniature greenhouse covers for seedlings
It is hard to keep a steady temperature inside them, which can do more harm than good.
- Step 19
Begin hardening off about two weeks before you plan to plant outdoors
Move seedlings to a protected spot outside that gets part sun, and reduce watering without letting the plants wilt.
- Step 20
Bring the plants back indoors each evening at first
Keep doing this until nighttime temperatures stay at or above 50°F (10°C), and keep seedlings out of wind or temperatures below 45°F (7°C) in the meantime.
- Step 21
Continue hardening off for two to three weeks total before transplanting
Or use a cold frame: take seedlings out by day and in at night for three to five days, depending on the plant and weather.
- Step 22
Increase the seedlings' sun exposure gradually
Give them shade or limit their time in direct sun at first, since a sudden jump to full sun can damage the plant.
- Step 23
Move seedlings to a bigger pot once they have one or two sets of true leaves
Choose a pot about one to two inches (2.5 to 5 cm) wider than the cell pack they came from.
- Step 24
Hold each seedling by its cotyledons, or seed leaves, never by the stem, when you move it
Handle it gently, since a seedling that loses its main growing point may not survive.
- Step 25
Tear off any peat that rises above the soil line on peat pots before planting
Leftover peat above the soil can wick essential moisture away from the plant.
- Step 26
Remove humidity domes once seedlings grow tall enough to touch them
If the mix settles more than half an inch (1.3 cm) from the rim, add more and water again.
- Step 27
Check seedlings for readiness before you move them to bigger pots or outdoors
Look for a few sets of leaves and well-developed roots, and skip any plant that has turned yellow, gone woody, or already started flowering.
- Step 28
Check whether a plant is root bound before transplanting it
Hold the main stem and gently tap the bottom of the pot to see the root pattern.
- Step 29
Water seedlings several hours before you transplant them outdoors
Then do the actual transplanting in the late afternoon or early evening, which helps prevent wilting.
- Step 30
Wait for the right outdoor conditions before transplanting warm-season crops such as tomatoes, squash, melons, peppers, and eggplants
Nighttime temperatures need to stay above 50°F (10°C), and soil needs to stay above 70°F (21°C).
- Step 31
Handle the transplants carefully to avoid damaging roots or bruising stems
Dig each planting hole wider and slightly deeper than the seedling's root ball.
- Step 32
Mix a starter solution into the soil at the bottom of the hole
Half-strength fish emulsion, finished manure, compost tea, or an organic fertilizer blend all work for this.
- Step 33
Set the transplant at the right depth
Line up the bottom leaves with the hole's top, except tomatoes, which can go deeper so only the top two or three leaf sets stay exposed.
- Step 34
Fill the hole to the top with tepid water once the transplant is in place
Then gently firm the soil around it with your fist.
- Step 35
Protect new transplants from wind and sun for the first few days
Give them extra water during their first week in the ground to help them get established.
Mistakes to avoid
If the seed-starting mix dries out during germination, the seed will no longer be viable. A sunny window is usually not enough, and the result is leggy seedlings. Bagged topsoil, vegetable gardening mixes, and potting mixes meant for larger plants are poor choices for starting seeds. Mixes with large particles like bark or wood chips should also be avoided. Sharing one flat among many seedlings lets neighboring roots grow into each other, and they are injured later at transplanting. Heavy watering is a problem when plants are small or when algae shows on the soil surface. Picking a seedling up by its stem is a mistake, since you should hold it by the cotyledons. A seedling that has lost its main growing point may not survive. Exposing seedlings to sunlight too quickly damages the plant, because the adjustment must be gradual. Wind and temperatures below 45 degrees F are hazards for tender seedlings during hardening off. Transplanting plants that are yellow, woody, already flowering, or root bound is best avoided. Buying more seed than you can use within two to three years is also best avoided. Seed that is properly stored can remain viable for two to ten years.
How this page was made
- Written with AI assistance from the sources listed on this page. AI is never the source of a fact.