When to start planting seeds indoors? Check your frost date

February marks the midpoint of winter, and with spring just over the horizon, many gardeners are dreaming of sunny days and dirty fingernails. For those looking to get a jump on the growing season, starting seeds indoors offers the most gratifying – and productive -- option.

Before digging in, however, it’s imperative to know exactly when to start, which means finding the last average frost date in your area. Consulting with your county’s cooperative extension office should arm you with hyper-local guidance. Absent that, there are some good online calculators that will get you close.

I like the one provided by The National Gardening Bureau (https://garden.org/apps/frost-dates/). To use it, plug in your zip code and take note of the date in the field where the “10%” column intersects with the “Last 36-Degree” row. That represents the date when on average, there is only a 10 percent chance the temperature will drop below 36 degrees, meaning outdoor transplantation is a safe gamble.

Armed with the scientific knowledge that water freezes at 32 degrees, some will lament my conservative recommendation. They may even point to years past when they’ve successfully transplanted tender seedlings outdoors weeks earlier. But as someone who has been burned by a late frost, I’m here to tell you that patience is key. Remember, those dates are averages, and temperature extremes can fluctuate wildly from year to year, past luck notwithstanding.

Those gardening in horticultural zones higher than 9 can ignore this advice, as the southernmost parts of the continental U.S., as well as Puerto Rico and Hawaii, are considered frost-free (there, it’s the heat of spring and summer that should be avoided.)

The rest of us will need to do some math. Seed packets typically advise that seeds be started a set number of weeks before the last frost. After calculating your start date, which will be unique for each seed type, it’s important to respect it. Start too soon, and plants likely will grow weak and struggle to thrive; start too late, and your harvest will be delayed, sometimes even inhibited from reaching its full potential.

While you’re waiting, obtain and prepare appropriate containers, which include multi-cell plug trays, individual biodegradable pots and recycled plastic yogurt containers. Each should have a drainage hole poked in its bottom. If planning to reuse last year’s vessels, be sure to disinfect them with a solution made by combining 9 parts water with 1 part chlorine bleach and rinse.

Fill each container or compartment with a sterile, moist, soilless seed-starting mix and, depending on seed size, sow one to four seeds per cell.

Place containers on a low-rimmed tray to which you’ve added no more than an inch of water, and monitor the soil’s moisture as it absorbs water through drainage holes. Replenish water as needed to keep the soil from drying out or becoming soggy. Bottom watering in this manner avoids accidentally washing away seeds, and reduces the occurrence of fungal diseases.

Cover the cells tightly with plastic wrap or a humidity dome, and place them in a warm spot out of direct sunlight. Heating mats placed under the trays will hasten germination.

Be on the lookout for “damping off,” a fungal disease that thrives in cool, damp, dark locations. If you spot a moldy, white layer on the soil’s surface, scrape it off with a spoon and allow the soil to dry completely between waterings. If there is a reoccurrence, dispose of the soil and seeds, disinfect the containers and start over with clean gear.

One way to help avoid damping off, as well as mold and algae growth, is to direct the breeze of a small fan to the soil. Later, that fan will train tender seedlings to withstand wind -- and they’ll grow sturdier in response.

When seedlings poke up, remove the dome or wrap, and place containers by a sunny window, or under fluorescent or LED grow lamps for 14 hours daily. Expensive lamp models aren’t necessary; ordinary shop lights will do. Keep the light source no more than 2-4 inches above the plants, adjusting its height as the seedlings grow.

If multiple seedlings sprout in each container, clip the weakest at the soil line using manicure scissors, retaining only one sturdy plant per cell. If allowed to remain, the roots of multiple seedlings will become entangled and threaten the viability of the young plants.

A week before the last frost date, begin to “harden off” plants by placing them outdoors for incrementally longer periods each day. Place them in a shady spot protected from wind, leave them there for one hour, then bring them back indoors. Repeat the next day, but leave them out for two hours, and continue adding an hour of outdoor exposure each day for a week, at which time your plants will be acclimated and ready to be planted in the garden.

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Jessica Damiano is an award-winning gardening writer, master gardener and educator. She writes The Weekly Dirt newsletter and creates an annual wall calendar of daily gardening tips. Send her a note at [email protected] and find her at jessicadamiano.com and on Instagram @JesDamiano.