Starting Seeds Indoors: How and When to Start Seeds
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A Guide to Starting Vegetable Seeds Indoors
January 25, 2023
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It’s seed-sowing time! But what’s the best way to start seeds? How do you even begin? In this article (with new video demonstration!), we’ll show the curious how it’s done! Let’s sow some seeds together, talk through the best ways of encouraging them to germinate, and expose a few common mistakes.
The Hindi word for seed is bija, which translates literally as “containment of life.” An apt description for 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 each week!
Why We Start Seeds Indoors
There are many benefits to sowing seeds indoors:
Obviously, it gives you a head start on the growing season, which can lead to more fruitful harvests.
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 Midwest), there are not enough growing days for those plants to get to harvest if they’re started outside. Starting seeds indoors allows you to gain 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-weather crops) before summer heat stifles growth.
If you don’t seed 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 of poor 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 day one.
There is a much wider range of varieties available as seeds—things you would never find in a six-pack at the local garden center!
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 plant them.
Finally, seeds are much less expensive than buying plants at the garden store.
It’s not just warm-season vegetables that can be started from seed. Many vegetables—such as carrots and radishes—do best when started from seed, as they dislike having their roots disturbed once they start growing. See our list of which seeds are best started indoors versus outdoors below.
Which Seeds Should You Start Indoors?
Not ALL seeds should be started indoors. In fact, most vegetables grow perfectly well when started outdoors and even prefer not to be transplanted. Ultimately, it’s important to consider how each type of vegetable grows in addition to where you’re growing it.
Consult the table below to see which crops are typically started indoors and which are typically started outdoors. Keep in mind that there isn’t a hard-and-fast rule about what you can start indoors and outdoors; it varies by your experience, your personal preference, your location, and the plant itself. In general, we find that:
Crops that are best started indoors include broccoli, brussels sprouts, cabbage, and tomatoes. Those with a slower root development, like cauliflower, celery, eggplant, and peppers, should also be started indoors.
Tender vegetables like 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 unpredictable weather.
Plants that do not transplant well and are therefore best started outdoors or in containers include cucumbers, muskmelon, pumpkins, squash, and watermelon. These are all tender, however, so refrain from sowing them outdoors while frost is still a threat.
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 seeds outdoors in the ground rather than transplant them later on. Plants with long tap roots also do not like to be transplanted; examples include dill and parsley.
Finally, plants like radishes and peas are so fast growing and cold tolerant that it just makes sense to get them right in the ground!
For seed-starting information customized to your location, check out our free online Planting Calendar.
It’s essential to sow seeds at the correct time. We don’t want to get too far ahead of ourselves! Sow too early and plants may have outgrown their pots before the weather has warmed up enough to plant them outside. But start seeds too late and they won’t have enough time to reach maturity before the end of the growing season. It’s a balance!
The Almanac’s Planting Calendar lists the ideal dates to start your vegetables. This customized tool is based on your zip code and local frost dates.
As a general rule, most annual vegetables should be sown indoors about six weeks before the last frost in your area. See local frost dates.
Your packet of seeds will often list when the seeds should be started indoors. For example, it may say, “start indoors 8 weeks before last expected frost date in your area.”
Our Garden Planner also has all the planting dates and aligned 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 sowing outdoors, as well as growing and even the harvest period!
Photo by Sergii Kononenko/Shutterstock
Equipment Needed for Seed-Starting
For starting seeds, you really only need a seed-starting mix, containers, and a good source of light.
Let’s start with our seed-starting mix. You can use either an all-purpose or multipurpose potting mix for big seeds such as beans and squash. But for finer seeds, we recommend a seed-starting mix (designed especially for starting seeds). We also like the seed-starting mix because it’s low in nutrients; the seed itself is already full of nutrients. Do make sure your mix is peat-free. (Extracting peat releases carbon and destroys natural habitat, so we prefer to use a more-sustainable alternative such as coconut coir.)
The right container: You can sow into pots/single seed trays or into plug trays/module trays. Each has its advantages:
Sowing into pots or a single tray container is more space efficient, as the young seedlings take up less space initially. It’s a more efficient use of seeds, too, because you can germinate the 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 seed flat. You can even stack trays up after sowing to save on space. After two or three days, start checking daily for signs of germination then move them out to the greenhouse or cold frame to continue growing. Or you can continue to grow seedlings on indoors, using grow lights to ensure strong, even growth.
Plug trays, on the other hand, are containers with individual pockets for each seed. They remove the need to transfer seedlings as often, minimizing root disturbance. Simply sow them into the plugs then grow them on 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 out. 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 or beetroot and salad onions.
We often repurpose food containers such as yogurt cups and sour cream containers as seed starting containers. Simply clean them out and poke a few drainage holes in their bottoms. 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 own pots.
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 biodegradable fiber. Using wide, shallow, flat containers to start rows of seedlings.
How to Sow Seeds
Sowing in a pot or a plug tray is really easy to do.
If sowing in a pot, fill it to the brim with the potting mix, then tamp 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.
Be sure to plant your seeds into the depressions at the depth listed on the seed packet. Most seeds can simply be gently pressed into the mixture with your fingers or you can use the eraser end of a pencil to do so. 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 salads, 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 later be thinned out). Larger seeds, like beans, are sown individually into deeper holes made with a finger, pencil, or dibber (a special seed-sowing tool).
Once done sowing, cover the seeds with potting mix so that they’re at the right depth (as listed on the seed packet).
Label your sowings, especially different varieties of the same type of plant. This is important! You might think you’ll remember, but it’s way too easy to get confused, particularly if you’ve got seedlings with very similar leaves. Pop on the date of sowing and the variety you’ve sown.
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 or young seedlings’ fragile roots. A mist sprayer is gentle, but can take a long time to actually get the mix properly saturated. Leave to drain through from the surface and then repeat. You really want to wet the mix, so 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 the bottom.
Watch this video to see the seed-starting advice in this article come to life. Ben will show you how it’s done, so you can sow like a pro!
Speeding Up Germination
We’re all impatient and want to see those seedlings push through as quickly as possible. The best way to achieve that is to give your seeds as close to ideal conditions as possible, which in most cases means a little warmth, so bring these early sowings indoors to germinate.
The seed packet should give an indication as to the ideal temperatures. In most instances, a tucked away corner of a warm room should work just fine, or on top of any appliance that gives off a little warmth—the top of fridges or freezers for example, or on a warm mantelpiece for warm-season crops like tomatoes, for instance.
Keep the potting mix from drying out and conditions nice and toasty by covering the plug tray or pot with some sort of clear cover. You could use a purpose-sold humidity dome or propagator lid for this. Or, just cover loosely with plastic and poke a few holes in the plastic with a toothpick for ventilation; mold growth can occur if containers are not allowed to “breath.”
Seed packets usually give you an idea of how long germination should take, but nevertheless, nothing beats regular inspections—and that’s half the fun anyhow!
Once around half of the seedlings are sprouted, remove them from the humidity dome or remove from the plastic covering; then move the seedlings to somewhere with good, strong light.
Hang the lights so they can be adjusted to keep them 4 inches above the plants as they grow.
Let There Be Light
A common mistake is to place seedlings on a windowsill, which rarely gives the same quality of light as outdoors. You can try turning seedlings daily to help them grow straighter, but more often than not the result is leggy seedlings that struggle to recover. Poor light levels are often the killer at this time of year, 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 grow lights.
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 department store.
With inexpensive grow lights, the lamp unit can be adjusted up or down. 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 more space.
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, 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 a timer.
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 light levels have improved just a bit more.
How to Transfer Seedlings
If you grew seedlings all together in a tray, you’ll often need to transplant—or prick out—seedlings into their own pots once they have germinated. The best time to do this is once they have two pairs of leaves—usually a set of seedling leaves and the first set of true or adult leaves. (See video below.) That being said, it’s fine to move seedlings on a lot earlier than this—as soon as the seedlings are big enough to handle. 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 or disease. It’s also a lot easier to separate seedlings out when they’re small.
To transplant seedlings, fill new pots with your potting mix; at this stage, an all-purpose or multipurpose potting mix is just fine. Make your holes then carefully remove the seedlings from their nursery pot. If you aren’t transferring all of the seedlings, then lift out only what you need.
Separate the seedlings out, then transfer them to their waiting holes. Only ever handle seedlings by their leaves; if you damage or crush the fragile stem, the seedling’s had it!
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 to damage.
You can set seedlings a little deeper if they are a bit leggy to help support them and get them back on track.
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 soon recover.
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 on Pricking Out, Potting On, & Transplanting.
Hardening Off Seedlings
Seedlings of tender crops must be gradually introduced to outside conditions before they are planted, a process known as “hardening off.” Suddenly moving plants from a stable environment to one with wide variations in temperature, light and wind can seriously weaken 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 less often.
About 7 to 10 days before transplanting, set the seedlings outdoors in dappled shade for a short time. Make sure the spot is sheltered from winds.
Start with an hour a day, then gradually extend the amount of time that plants are outside, until they’re staying out all day.
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 too harsh.
Cool-season crops don’t really need as much hardening off. 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 need acclimatizing.
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 for this.
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 their containers.
See our video on How to Harden Off Plants.
If you’re not able to be around to bring your seedings 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 and Tips
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 your area.
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 Beginners for popular beginner vegetables.
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 or birds? That’s why it’s a good idea to sow seeds in small batches a few weeks apart.
It’s worth making an early start, because if you lose those seedlings it doesn’t matter so much; you can always sow some more.
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 plant out.
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 potting mix!
Put in place pest prevention techniques—row covers like netting or fleece for bird protection, and slug traps.
Many quick-growing crops are harvested often. Take lettuces or radishes as an example. Sow a small 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 planning!
More Reference Material
Consult the Almanac’s library of Growing Guides, which provide planting, care, and harvesting information for each of the common vegetables, fruit, and herbs.
We’ve also gathered all of our best beginner gardening articles in one place with the Learn to Garden series. From selecting the right gardening spot to choosing the best vegetables to grow, our Almanac gardening experts are excited to teach gardening to everyone—whether it’s your 1st or 40th garden.