When to plant indoors?

How do I know when to start seedlings indoors?

Growing plants from seed is fun and can be an antidote for the winter blues and cabin fever. It can also save you money and allow you to grow unique varieties that aren’t readily available at garden centers. One of the best sources of information about starting seeds indoors can be found right on the seed packet. No matter which company your seed is from, you will find information on the label about days to germination and harvest, depth to plant, optimal soil temperature for germination, seed spacing, and how many weeks before the last frost to start indoors.

In Zone 5, where most New Hampshire residents live, the last frost is typically in late May. That means you can anticipate planting your tender flowers and vegetables in the garden after Memorial Day. Of course this isn’t a hard and fast rule and you should always pay attention to long-range forecasts. If you want to start tomatoes, the package will direct you to start the seeds indoors six to eight weeks before the last frost. If you want to plan to transplant your flowers and vegetables into the garden on May 27th, then you would start your seeds between April 1st and April 15th. Cucumbers can be started four to six weeks before planting date, while peppers and eggplant may require at least 10 weeks of time indoors.

Although you may be anxious to get your seeds going, don’t be too hasty. Plants that are started from seed too early will probably be of poor health and quality by the time you’re ready to plant them in the garden. Instead of rushing to sow, get out your seed packets and make a garden plan!

Planting Vegetables from Seed and Seedling

By The National Gardening Association, Bob Beckstrom, Karan Davis Cutler, Kathleen Fisher, Phillip Giroux, Judy Glattstein, Michael MacCaskey, Bill Marken, Charlie Nardozzi, Sally Roth, Marcia Tatroe, Lance Walheim, Ann Whitman

You can plant vegetable seeds indoors or outdoors. If you plant seeds indoors, you transplant them into your garden later. With direct seeding, you skip the indoor step and sow the seeds directly in your garden. If you’re serious about growing vegetables, you’ll probably end up using both options. Consider these points when making your choice:

  • You get a jump on the growing season when you sow seeds indoors. This process is called seed starting (or starting, for short). If you start at the right time, you can have vigorous seedlings ready to go into the ground at the ideal time. In areas with short growing seasons, starting seedlings indoors really gives you a head start.
    The best candidates for an early start are plants that tolerate root disturbance and benefit from a jump on the season, including broccoli, Brussels sprouts, cabbage, cauliflower, celery, eggplant, leeks, onions, parsley, peppers, and tomatoes.
  • Seeds are easier to start indoors than outdoors. You can more easily provide the perfect conditions for hard-to-germinate or very small seeds, including the ideal temperature, moisture, and fertility.
  • Some vegetables don’t like to be transplanted. These vegetables include many of the root crops, such as carrots, beets, turnips, and parsnips. They’re cold-hardy vegetables, so you can direct seed them pretty early anyway. Crops like corn, beans, and peas are also pretty finicky about transplanting and grow better when you direct-seed.

Transplanting seedlings into the ground

Harden off vegetable seedlings that have been grown indoors or purchased from a greenhouse before exposing them to the elements. Hardening off is a way of increasing your plant’s stamina before planting — similar to slowly acquiring a base tan before taking that outdoor, tropical vacation. Plants that have been growing outside at the nursery can go right into the ground, but greenhouse-grown plants are lush and soft and have never known a single day of sunshine in their lifetimes. You have to introduce them slowly to the harsh, real world.

To harden-off seedlings, leave the plants in their containers and put them in a shaded area with some indirect light for a few days. A north-facing, covered porch is ideal. Whenever a freeze is predicted, bring the plants inside overnight. If these are shade plants, you can leave them in this protected site for a few more days and then put them in the garden. For sunny-spot plants, give them a few days in the shaded area and then place the plants in a sunny location for an hour one day. Give them a couple of hours of sun the next day, and so on, increasing their exposure each day. At the end of a week, the plants are thoroughly accustomed to sunlight and wind and are ready to go into their new home.

Don’t overharden your plants. Certain crops, such as cabbage and broccoli, can bolt (flower before they’re supposed to) quickly if seedlings over three weeks old are repeatedly exposed to temperatures lower than 40°F (4°C) for a couple of weeks.

Before transplanting your seedlings, you need to prepare your soil and sculpt beds or rows, and your garden must be ready to plant. When setting out plants in biodegradable peat pots, make slits down the sides of the pots or gently tear the sides to enable the roots to push through. Also, tear off the lip (top) of the pot, so that it doesn’t stick up above the soil surface and pull moisture out of the soil. With premade growing blocks encased in netting, cut off the netting before planting.

Choose a calm, cloudy day to transplant, if possible. Late afternoon is a good time because plants can recover from the shock of transplanting without sitting in the midday heat and sun. If you don’t get an ideal transplanting day and the weather is hot and sunny, shade the plants until the sun goes down. Don’t be alarmed if your plants look a little droopy after you set them out because they’ll soon recover. Cabbage seedlings can droop and look almost dead, for example, and then be up and growing in a day or two.

Sowing seeds directly in your garden

Unless you live in an area where summers are really short, you’re better off sowing some types of vegetables directly in a garden. Large-seeded, fast-growing vegetables, such as corn, melons, squash, beans, and peas, usually languish if they’re grown in containers for even a day or two too long.

Before direct seeding, make sure that the soil has dried out sufficiently before you work it, and be sure that the soil is warm enough for the seeds that you want to plant. Pea seeds, for example, germinate in soil as cool as 40°F (4°C), and you can plant them as soon as you can work the soil in spring. Squash seeds, on the other hand, need warmth. If your soil temperature is much below 65°F (18°C), the seeds are likely to rot in the ground before they sprout. The best way to determine the temperature of your soil is to use a soil thermometer, which you can buy at a garden store.

You can plant seeds in a variety of patterns. The method that you choose depends on your climate, your tools, and your taste:

  • Row planting: Mark the placement of a row within your garden, and then make a furrow at the correct depth along the row. Some seeds may not sprout, so sow seeds more thickly than you want the final spacing of the crops to be. Thinning rows is less of a chore if you space seeds as evenly as possible. Cover the seeds with fine soil and then firm them in with the back of a hoe to make sure that all the seeds are in contact with the soil. Water gently. If you plan to use furrow irrigation, fill the furrows with water first and then push the large seeds into the top of raised beds.
  • Wide row planting: This method allows you to plant more seeds in less space by concentrating watering, weeding, and fertilizing in a smaller area. Rows are generally 10 to 16 inches (25 to 41 cm) wide. Sprinkle seeds over the entire row — with most crops, try to land the seeds about 1/2 to 1 inch (1 to 2 cm) apart. For peas and beans, space them 1-1/2 to 2 inches (4 to 5 cm). Cover small seeds with a thin layer of potting soil. Lightly pat the potting soil down again to bring the added soil into firm contact with the seeds.

  • Bed planting: Planting in beds is essentially the same as planting wide rows.
  • Hill planting: Plant seeds for vining crops that spread out, such as squash, melons, or cucumbers, in hills or circular groups. Loosen the soil in a 1-foot-diameter (30 cm) area, level the area, and then plant five to six seeds close together. Thin out all but the two strongest seedlings.
    If your soil is heavy, you may want to plant in a raised hill, or mound. The raised soil warms up more quickly than the surrounding soil and drains better. Just don’t let the mound dry out!

Soon after seedlings grow their second set of true leaves, you need to thin them out to avoid overcrowding. (The first set of leaves that a seedling produces are called seed leaves or cotyledon, which are followed by the true leaves.) When you thin plants, either discard the extra seedlings or move them to another part of your garden.

Newly transplanted seedlings need extra attention until they get established. Shade them from the hot sun for a day or two and be sure to keep them well watered.

Starting Seeds Indoors

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Starting seeds indoors is about as much fun as a food gardener can have in late winter! The idea is to grow baby plants (a.k.a. transplants or starts) for 2-8 weeks (depending on the vegetable and rate of plant growth) and then plant outdoors where the crops will mature and be harvested. Just about any crop can be started inside and transplanted outside. With a small investment and bit of space, you can grow hundreds of healthy transplants. All of the supplies you need can be found at home (reused food containers for starting seeds) or purchased locally from hardware stores, garden centers, and big box stores.

Growing Your own Transplants from Seeds

  • saves you money — this may take a few years since there are first-year set-up costs
  • increases your garden’s output — get earlier harvests by starting with transplants instead of seeds
  • allows you to grow the crops and cultivars you like best rather when you need them — no need to plant only what’s available in retail stores
  • gives you better control of germination and plant stand — fewer skips, no thinning
  • lessens pest and weather risks — no worries about cool, wet weather keeping you from planting or encouraging seed rotting diseases.

When to Plant Vegetable Seeds Indoors

The proper time to sow seeds for transplants depends on when plants may safely be moved out-of-doors in your area. This period may range from 2-3 weeks (lettuce) to 8 weeks (pepper, eggplant) before transplanting, depending on the speed of germination and rate of growth (see “Germination Information for Selected Vegetable Crops” below).
A common mistake is to sow seeds too early and then attempt to hold the seedlings back under poor light or improper temperature ranges. This can result in tall, weak, spindly plants that do not perform well in the garden. Sow tomatoes 6-7 weeks before you expect to plant. You will end up with stocky 8-10 in. tall plants. If they do get too tall, you can lay them down in a trench when planting and turn the growing tip up so only the top 2-3 sets of leaves is above the soil.

What to Plant Indoors

  • Typical vegetable transplants found in garden centers in the spring include cabbage, broccoli, and cauliflower; followed by tomato, pepper, eggplant, squash, cucumber, melon, and lots of different herbs. You can grow all of these under your fluorescent lights PLUS the following: beets, spinach, Swiss chard, kale, mustard, broccoli raab, arugula, Asian greens, onion, leek, bean, and sweet corn!
  • Don’t forget about your need for mid-summer, late summer and early fall transplants to keep your garden productive. It’s difficult to find vegetable transplants in stores beyond mid-June.
  • Use fresh seed or seed that has been stored properly from last year. Surplus seeds should be stored in a cool, dry location, like your freezer.

How to Plant Your Seeds

You can grow two standard size (10.5 in. X 21 in.) trays under one 4 ft. long fluorescent fixture with two tubes. You can grow four trays under two fixtures. Each tray can hold about 12-18 large transplants and 50-120 small transplants.

Set up your lighting system

  • Read about options for grow lights.

Choose containers and prepare growing medium

  • Choose your containers and growing medium.
  • Place your dry growing medium into a bucket or tub. Pour lukewarm tap water in slowly and mix it into the growing medium. It should be as moist as a wrung-out sponge, not soppy wet. Water sprinkled on top of dry soilless media will bead up and fail to soak in.


Dry media in wheelbarrow

Add water


Mix thoroughly


Moist soil ball

Fill containers

  • Fill your container to within ¾ of an inch from the top with the moistened medium.


Add media to container

Black flat filled with potting media

  • Firm the medium very lightly at the corners and edges with your fingers or a block of wood to provide a uniform, flat surface.

Mark your rows


Mark rows in flat with straight edge

  • Good light and air movement results from sowing in rows, as compared to broadcasting the seed randomly across the surface. If “damping-off” disease appears, there is less chance it will spread. Seedlings grown in rows are easier to label and handle at transplanting time.
  • For small-to-medium size seeds (all crops except cucumber, squash, melons, corn, beans) make rows about 1- to 2-inches apart and 1/8”-1/4” of an inch deep across the surface of the container, using a narrow board or pot label.

Plant

Planting seeds by hand
Planting seeds using folded index card

Lightly cover seeds with media

  • Sow the seeds thinly and uniformly in the rows by gently tapping the packet of seed as it is moved along the row. Lightly cover the seeds (lettuce seeds can be left on the surface un-covered or very lightly covered) and press down gently to ensure good contact between the seed and the soilless growing medium.
  • A suitable planting depth is usually about twice the diameter of the seed.
  • Sow large seeds (cucumber, squash, melons, corn, bean) directly into small containers or cell packs, eliminating the need for potting up latter. Sow two or three seeds per unit and later thin to allow the strongest seedling to grow.

Top 2 rows correctly spaced

Hand pull seedlings to correct spacing

Tips for quick germination

  • The seeds and growing medium need to be moist and warm to germinate. Generally, 65°-75°F is best for germinating seeds of most plants. This should be the temperature of the growing medium, not the air.
  • Seed germination begins with the absorption of water. An adequate and continuous supply is essential. Once the process has begun, a dry period will cause the death of the embryo. Spray some water from a plastic mister on the growing medium as needed, to keep it moist.
  • Cover the container with a piece of clear plastic or insert the container in a plastic bag. This will increase the humidity and temperature. The plastic should not be in contact with the growing medium. Remove the plastic as soon as sprouts appear.
  • You can buy heating pads to set your containers on to warm the growing medium and speed-up germination. A cheaper and easier approach is to drape clear plastic over your light fixture. The plastic should rest on the frame holding the fixture and not on the fixture itself. Leave the lights on and the heat from the ballast will be trapped inside the plastic tent and keep the temperature at 70º-75º F.
  • The top of your refrigerator is another good warm place for quicker germination.

Planting information on back of seed packet

Arugula seedling in yogurt cup

Broccoli seedling in 4-cell pack

Lettuce seedlings planted in rows

Germination information for selected plants

Plant Approximate
time to seed
before last frost
date (weeks)
Time seeds
need to
germinate
(days)
Temperature
(F)
Light/dark
requirement
Broccoli 8 5 to 10 70 Either
Cabbage 8 5 to 10 70 Either
Cauliflower 8 5 to 10 70 Either
Cucumber 4 or less 5 to 10 85 Either
Eggplant 8 5 to 10 70 Either
Lettuce 8 5 to 10 70 Light
Muskmelon 4 or less 5 to 10 85 Either
Pepper 8 5 to 10 80 Either
Squash 4 or less 5 to 10 85 Either
Tomato 6 5 to 10 80 Either
Watermelon 6 5 to 10 70 Either

Need help? Ask the Experts when you have questions or problems.

  • Seedling Care (watering, transplanting to larger containers)

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