Starting Seeds Indoors: How and When to Start Seeds (2024)

When do you start your seeds indoors? Sow too early, and the plants may be ready to leave the pot before it’s warm enough. Sow seeds too late, and they won’t have enough time to reach maturity. We’ll provide some tips as well as a chart on how many weeks to sow indoors before last springfrost.

The Hindi word for seed is bija, which translates literally to “containment of life.” This is an apt description of these tiny miracles that contain everything needed to make a new plant. This time of year, we are up to our elbows in dirt, starting more seeds indoors eachweek!

Why We Start SeedsIndoors

There are many benefits to sowing seedsindoors:

1. Obviously, it gives you a head start on the growing season, which can lead to more fruitfulharvests.
2. It’s actually necessary for a number of plants. Warm-season vegetables—such as tomatoes, peppers, and eggplant—can’t be planted too early in the spring, as the soil is too cool. In many regions (including New England and the Midwest), there are not enough growing days for those plants to get to harvest if they’re started outside. Starting seeds indoorsallows you togain a few precious weeks of growing time, which can really make a difference.In warmer regions, starting seeds indoors can allow you to get in an extra round of crops (especially cool-season crops) before summer heat stiflesgrowth.
3. If you don’t start seeds indoors, you will need to buy young plants called “transplants” or “starts” at the garden store or nursery. While some nursery starter plants are grown nicely, others may be ofpoor quality and don’t thrive once they’re home. When you plant your own seeds, you tend to have healthier starts since you can care for them from dayone.
4. A much wider range of varieties is available as seeds—things you would never find in a six-pack at the local gardencenter!
5. You will know how they have been raised—organically instead of bathed in a wash of chemicals. You can time the plants to be ready for when you want to plantthem.
6. Finally, seeds are much less expensive than buying plants at the gardenstore.

When to Start SeedsIndoors

As a general rule, seeds are started indoors about six weeks prior to your last frostdate.

1. Read your seed packet. Most will list when the seedsshould be started indoors (or outdoors). For example, it may say,”Start indoors 8-10 weeks before the last expected frost date in your area.” You can simply count back from your frost-free date by checking the Almanac’s Frost Date Calculator.
2. If you don’t have your seed packets yet, the Almanac’sPlanting Calendarcalculatesideal dates to start seeds for common vegetables and herbs based on your frost date and yourlocation
3. Finally, if you plant your garden with the Almanac online Garden Planner also has all the planting dates and aligns with your entire garden plan for the season. The Garden Planner looks up climate data from your nearest weather station and then uses that to calculate the best range of planting dates for each crop in your plan. It’s nicely color-coded to show you dates for sowing indoors and outdoors, as well as growing and even the harvestperiod!

Which Seeds to StartIndoors?

Not ALL seeds should be started indoors. In fact, most vegetablesgrow perfectly wellwhen started outdoorsand even prefer not to be transplanted. Ultimately, it’simportant to consider how each type of vegetable grows in addition to where you’re growingit.

Consult the table below to seewhich cropsare typically started indoors andwhich are typically startedoutdoors.Remember that there isn’t a hard-and-fast rule about what you can start indoors and outdoors; it varies by your experience, personal preference, location, and the plant itself. In general, we findthat:

• Crops that are best started indoorsincludebroccoli, brussels sprouts, cabbage, and tomatoes.
• Those with slower root development, likecauliflower, celery, eggplant, and peppers, should also be startedindoors.
• Tender vegetableslike tomatoes, eggplants, and peppers are very susceptible to the cold temperatures of spring, so it’s best to start them indoors and keep them safe from unpredictableweather.
• Plants that do not transplant well and are, therefore, best started in the garden (or in outdoor containers) include cucumbers, melons, pumpkins, and squash. These are all tender, however, so refrain from sowing them outdoors while frost is still athreat.
• Some plants truly resist transplanting. Root vegetables, like carrots, turnips, and beets, don’t like having their roots disturbed, so it’s usually safer to just start their seedsoutdoors in the ground rather than transplant them later on. Plants with long tap roots also dislike being transplanted; examples includedilland parsley.
• Finally, plants like radishes and peas are so fast-growing and cold-tolerant that putting them right in the ground makessense!

More Accurate Timing: SoilTemperature!

Best Soil Temperature for StartingSeeds

Most vegetable crops have a minimum germination temperature between 36°F and 60°F (2°C and 16°C), but there is also an optimal range. This is where the difference between cool-season crops (spinach, lettuce, cabbage, etc.) and warm-season crops (eggplant, cucumbers, tomatoes, peppers) comes into play. For example, parsnips will germinate best between 50°F and 70°F (10°C and 21°C), but eggplant will germinate best between 75°F and 90°F (24°C and 32°C), tomatoes between 61°F and 86°F (16°C and 30°C), and peppers between 64°F and 95°F (18°C and35°C).

How to Start Seeds: 3Steps

For starting seeds, you only need a seed-starting mix, containers, and a strong light source (but more on thatlater).

1. Choosing a PottingMix

Let’s start with the potting mix. Generally, you will have no trouble using an all-purpose potting mix. Drainage is good, but if your potting mix seems to have a lot of big chunks of wood or rocks, we recommend sifting it through a screen before using it for seed starting. Seedlings’ roots will struggle in soil that’s not fineenough.

For very small or delicate seeds, we recommend using an actual seed-starting mix, which is a potting mix designed especially for starting seeds. We also like seed-starting mixes because they’re low in nutrients; the seed itself is already full of nutrients. Avoid peat-based mixes, if possible. We use more sustainable alternatives, such as coconutcoir.

Do NOT use soil from outdoors. It’s often too heavy or compacted for seedlings’ roots and may contain pests ordiseases!

2. Choosing aContainer

You can sow into pots, plug trays, or recycled containers. Each has itsadvantages:

• Sowing many seeds into a large pot is space efficient, as the young seedlings take up less space initially. It’s a more efficient use of seeds, too, because you can germinate many seeds in a pot and then transfer every single seedling into its own pot or plug. Sowing into a single container can also be useful for sowing very tiny seeds such as basil or easy-to-transplant flower seeds. For easy, cool-season crops—everything from onions to celery to cabbage—you can sow multiple seeds in the same container.
  
• Plug trays, on the other hand, are flatter containers with individual pockets (or “cells”) for each seed. They remove the need to transfer seedlings as often, minimizing root disturbance. Simply sow them into the plugs, then grow them until it’s time to plant them, though they may need transplanting into bigger plugs or pots if the roots fill their plugs before it’s time to plant them outside. Two or more seeds are usually sown per plug, and then the germinated seedlings are either left to grow on as a cluster or thinned out to leave the strongest seedling in each plug. Trays with smaller plugs suit most leafy greens and radishes, especially if they will be transplanted promptly (within three or four weeks of sowing). We also like this method for cluster-grown crops such as beets, beetroot, and salad onions.
  
• Recycled containers: We often repurpose food containers such as yogurt cups, sour cream containers, or plastic muffin trays as seed starting containers. Clean them out and poke a few drainage holes in their undersides. They are generally large enough to house one or two small seedlings for a few weeks. Eventually, seedlings will need to be transplanted into their ownpots.

Well-made containers and trays of rigid plastic can potentially last for many years, but if you want to avoid plastic, look for alternatives made of biodegradablefiber.

3. Sowing in thePot

Sowing in a pot or a plug tray is really easy todo!

1. If sowing in a pot, fill it to the brim with the potting mix, then tamp it down to a firm level. It’s hard to over-firm, and seedlings prefer plenty of potting mix to sustain them. If you are using plug trays, fill the plug trays right to the top, then tamp down to settle. Top up with a little more of the mix, then brush off the excess.
   
2. Use your finger or the eraser-end of a pencil to poke planting holes in the mix. Be sure to sow the seeds at the depth listed on the seed packet. Many seeds can simply be gently pressed into the mixture with your fingers, too.When choosing which seeds to plant, choose the largest, healthiest-looking seeds in the packet for the best chance at germination. Many vegetables, including common crops such as salad greens, onions, beets, peas, and radishes, may be sown in pinches of three to five seeds per plug for planting out as a cluster of seedlings (to be later thinned out as they grow). Larger seeds, like beans, are sown individually into deeper holes made with a finger, pencil, or dibber (a special seed-sowing tool).
   
3. Once done with sowing, cover the seeds with potting mix so that they’re at the right depth (as listed on the seed packet).
   
4. Label your sowings, especially different varieties of the same type of plant. This is important! You might think you’ll remember, but it’s too easy to get confused, particularly if you’ve got seedlings with similar leaves (such as multiple varieties of tomatoes). Note the date of sowing and the variety you’ve sown.
   
5. Water the pots or trays carefully using a watering can fitted with a fine sprinkling rose or a clean turkey baster. A pitcher may let the water out too forcefully, dislodging the seeds. A mist sprayer is gentle but can take a long time to actually get the mix properly saturated. After watering, leave the mix to drain through from the surface and then repeat. You really want to wet the mix at the beginning so that the seeds are woken up from their slumber! Don’t worry; if it’s a good mix, it’s hard to overwater at this point; any excess will just drain out of thebottom.

Watch this video to see the seed-starting advice in this article come to life. Ben will show you how it’s done so that you can sow like apro!

Tips to Speed UpGermination

We’re all impatient and want to see those seedlings push through quickly! The best way to achieve that is to give your seeds as close to ideal conditions as possible, which usually means a littlewarmth.

• The seed packet should give an indication as to the ideal germination temperature. In most instances, a tucked-away corner of a warm room should work justfine.
• To further speed up germination, you could use a heating pad to warm the soil under your seedlings. Just leave enough space between it and your seedlings so you don’t bakethem!
• Keep the potting mix from drying out and conditions nice and toasty by loosely covering the plug tray or pot with plastic wrap; poke a few holes in the plastic with a toothpick for ventilation; mold growth can occur if containers are not allowed to“breathe.”
• After about half of the seedlings are sprouted, remove them from the humidity dome or remove them from the plastic covering; then move the seedlings to somewhere with good, stronglight.

See our article on “How to Improve Germination.”

Add GrowLights

Poor light levels are often the killer in wintertime, rather than cold, at least for cool-season crops. So, if it’s early in the season and you don’t have a suitable outdoor protected structure such as a greenhouse or cold frame, it might be worth investing in some full-spectrum growlights.

Placing seedlings on a windowsill rarely gives the same light as outdoors. You can try turning seedlings daily to help them grow more upright, but more often than not, the result is leggy seedlings that are bent in all directions and that will struggle torecover.

• Grow lights don’t need to be anything particularly fancy. An LED or fluorescent light fixture that’s “full-spectrum” (i.e., produces light in the full range of the visible spectrum—like the Sun) can usually be found for under $40 at a local hardware or departmentstore.
• The lamp unit can be raised up and down with inexpensive grow lights. Ideally, you want the lights to be about 4 to 6 inches above the canopy of the seedlings. That’s far enough above not to be too warm but to give a good, strong light. Move the lamp unit up as the plants grow and need morespace.
• Up to 16 hours a day of light is fine. In fact, in most cases, the longer you leave them on, the quicker seedlings will grow, so this is a good way to catch up on growth early on in the season. (Note that plants do need a period of darkness, too, so don’t leave your lights on 24/7!) Many gardeners switch lights on when getting up in the morning and then switch them off when heading off to bed, which means they’re on for around 15 to 16 hours. Or you could, of course, put your grow lights on atimer.
• Plants can move from grow lights to outdoors or under protection outside when it’s either warm enough or, more usually, for cool-season crops once the outdoor light levels have improvedslightly.

See our articles on using grow lights and the best grow lights for growing vegetables indoors.

Next Step: TransferSeedlings

After you start your seedlings and they have two pairs of leaves, you’ll often need to transplant them into their own pots. Don’t delay transplanting your seedlings, as you don’t want them to become overcrowded, which can cause all sorts of issues, such as leggy seedlings ordisease.

• To transplant seedlings, fill new pots with your potting mix; at this stage, an all-purpose or multipurpose potting mix is ideal. Make your planting holes, then carefully remove the seedlings from their nursery pots. If you aren’t transferring all the seedlings, lift out only what youneed.
• Separate the seedlings out, then transfer them to their planting holes. Only ever handle seedlings by their leaves; if you damage or crush the fragile stem or roots, the seedlings are donefor!
• Try to avoid damaging the roots as much as possible, too. One way to do this is to bring along as much of the potting mix that’s around the roots as you can. This is one reason, in fact, why working with really young, small seedlings is often better: they are really quick-growing, and their roots are nowhere near as extensive as more established seedlings, so there’s less root todamage.
• You can set seedlings deeper than they had been growing, especially if they are slightly leggy. This helps support their stems and get them back ontrack.
• Firm in around seedlings. Once you’re done, gently water the seedlings with a watering can. Don’t worry too much if the seedlings get a little flattened; they’ll soonrecover.
• Check your seedlings and plants regularly for soil moisture. Push a thumb into the potting mix or simply lift the pot up to gauge how heavy it is. You’ll get a feel for this with more experience, but the heavier it is, the more water it will contain and the less likely it is to need watering.

  See our video below on how to transplantseeds.

Ready for Outdoors? HardenOff!

Seedlings of tender crops must be gradually introduced to outside conditions before they areplanted in the garden, a process known as “hardening off.” Suddenly moving plants from a stable indoor environment to one with wide variations in temperature, light, and wind can seriously weaken—orkill—plants!

For most plants, start hardening off about 7 to 10 days before the final frost date for your area. Check our Planting Calendar for safe dates to plant outside and work back from there. Withhold fertilizer and water them a little less often during thisperiod.

Here’s how to harden off yourseedlings:

• About 7 to 10 days before transplanting, set the seedlings outdoors in dappled shade for a short time each day. Make sure the spot is sheltered fromwinds.
• Start with an hour a day, then gradually extend the amount of time that plants are outside until they’re staying out allday.
• Keep the soil moist at all times during this period. Dry air and spring breezes can result in rapid transpiration. If possible, transplant on overcast days or in the early morning when the sun won’t be tooharsh.
• Cool-season crops don’t really need as much hardening. Crops such as lettuce, onions, beets, or peas can go straight outside as soon as the ground is ready, meaning that the soil is no longer cold and wet and has reached around 50ºF (10ºC). But warm-season crops such as tomatoes and peppers will needacclimatizing.
• A great way to toughen up plants—whether indoors or under cover in a greenhouse or cold frame—is to run your fingers lightly over the foliage. This mimics wind to create sturdier plants. Indoors, you could also use a fan forthis.
• If outdoor conditions allow, plant seedlings out while they are still quite young—sometimes as soon as 3 to 4 weeks after first sowing. Younger seedlings tend to establish quicker than those that have become root-bound in theircontainers.

See our video on How to Harden Off Plants.

If you’re not able to be around to bring your seedlings back and forth from the outdoors, another option is to place your seedlings into a cold frame and gradually increase the amount of ventilation by opening vents progressively wider each day. Make sure to shut them down completely before dark. (See how to make a cold frame for cold-weather protection.)

Final Thoughts andTips

• Be seed-savvy. Obtain seed catalogs from several companies and compare their offering and prices. Some of the regional companies may carry varieties better suited to yourarea.
• Make a list of what you’d like to grow. A good rule of thumb is to imagine your garden one-quarter the size that it really is. This allows for good spacing practices! See Vegetable Gardening for Beginnersfor popular beginnervegetables.
• Experienced gardeners always hedge their bets and prepare for some losses. Successful gardening depends on so many factors. Is the season unusually warm or cold this year? Are your first seedlings going to be eaten by pests like slugs, birds, or rabbits? That’s why it’s a good idea to sow seeds in small batches a few weeksapart.
• If you’re starting out, it’s worth starting a little early because losing those seedlings doesn’t matter so much; you can always sow somemore!
• The Garden Planner shows you exactly how many plants you’ll need based on your layout. But, hey, sow a few more than you’ll need as spares, just in case, or so you can select the very biggest, healthiest seedlings to plantout.
• If you have extra seeds that you’ll be pouring back into the packet, do this over the pot you’ve just sown. That way, if you drop any, they’ll end up sown with the right batch of seeds rather than being wasted or mixed up in the spare pottingmix!
• Put in place pest-prevention techniques, such as row covers like netting or fleece for bird protection and slug traps.
• Many quick-growing crops are harvested throughout the growing season. Take lettuce or radishes as an example. Sow a new plug tray every couple of weeks throughout the growing season, and that way, you can look forward to a succession of harvests rather than them all coming at once. That’s smart gardenplanning!

More ReferenceMaterial

Consult the Almanac’s library ofVegetable Growing Guides, which providesplanting, care, and harvesting information for each of the common vegetables, fruits, andherbs.

What seeds are you starting thisseason?