How to start flower and vegetable seeds indoors
In today's "Growing Together" column, Don Kinzler says some varieties should be planted soon to get ready for the upcoming gardening season.
You can always spot a gardener’s garage, because an entire section is dedicated to used cell packs, plastic nursery pots and stacks of old greenhouse trays that we’re saving in case we need them for something someday. Starting your own plants from seed is the perfect occasion to repurpose these supplies.
Some vegetable and flower types are easier to grow from seed than others because they sprout quickly, emerge strongly and don’t require as much indoor growing time. In the easier category are marigold, zinnia, cleome, alyssum, cosmos, calendula, four o’clock, nasturtium, tomato, pepper, cabbage, cauliflower, broccoli, melons, cucumber and squash. Flowers that are slightly more difficult and require more weeks to develop include petunia, coleus, impatiens, salvia and moss rose.
The overall plan is to sow the seeds, coax them to germinate, grow them in the seed tray until they’re large enough to handle, and then transplant each seedling into individual pots, cups or cell packs.
Why not plant the seeds directly into their final packs or pots? Why go through the labor of transplanting the little seedlings? Most seeds require specific temperatures to coax germination, and it’s easier to give these ideal conditions to a smaller seed tray than a larger assembly of pots or packs. Transplanting seedlings also imparts vigor and creates sturdier plants.
It’s best not to start too early. Suggested seeding dates are as follows:
- March 1: impatiens, petunia, snapdragon and lobelia.
- March 15: alyssum, dianthus, salvia, broccoli, cabbage, pepper and eggplant.
- April 1: tomato, cleome, marigold and lettuce.
- April 15: cosmos, calendula, nasturtium, four o’clock and zinnia.
- May 1: squash, melons and cucumber.
Materials needed: Use mix that’s labeled for starting seeds instead of all-purpose potting mix, because seed-starting mix is milled finer. Containers should be at least 2 inches deep with drain holes drilled or punched. Clear plastic trays from the grocery bakery or deli work well, and holes are easily added.
- Moisten the seeding mix the day before using by adding water to the bag and mixing well. Dry mix is difficult to moisten after seeding, and seeds will often float to the surface.
- Fill container to the top with mix. It will settle slightly when watered. Use a separate container for each seed type, because different types germinate at different rates.
- Seeds can be broadcast over the surface or planted in rows.
- Planting depth is shallower than we might think. Small seeds the size of a poppy seed should be sprinkled on the surface without covering with mix. Cover larger seeds to a depth about twice the seed’s diameter.
- Label with variety and date.
- Water gently with a fine mist or sprinkling-type watering can. Use warm water to stimulate seed growth.
- Cover the container with clear plastic wrap or a clear lid to conserve humidity and warmth. If watered well following seed sowing, trays usually won’t require additional sprinkling until seeds have sprouted.
- Place seed trays in a warm location so the mix stays between 70 and 78 degrees. Electric seeding mats, available at garden centers, work wonders for coaxing seeds to sprout uniformly and rapidly.
- Most seed types require seven to 10 days to germinate. As soon as seedlings are visible, move the container to a sunny window or under electric lights. To better eliminate the chance that newly sprouting seedlings will stretch and become spindly, locate the seed tray in bright light immediately after sowing.
- When a majority of the seeds have sprouted, remove the container’s clear cover and remove from the heat mat, if used.
- If growing seedlings under fluorescent or LED light fixtures, set a timer for 16 hours of light and eight hours of darkness.
- The best temperatures for “growing on” are 65 to 70 degrees. Let the surface of the mix dry between gentle waterings.
- When seedlings are just large enough to handle easily, transplant from the seed tray into individual cell packs or pots for continued growth.
Don Kinzler, a lifelong gardener, is the horticulturist with North Dakota State University Extension for Cass County. Readers can reach him at firstname.lastname@example.org.