Critical Trio of Seed Starting: Water, Light, and Heat

Heat mats are a good investment

By Lorna Carlisle
Lorna Carlisle, a Beacon volunteer, discusses how to start seeds. She recommends using a heat mat. Photo: Lorna Carlisle

When you start seeds, three things are critical: water, light, and heat.  I am a firm believer in a heat mat if you can afford one. They come in various sizes but a  21” x 48” mat is around $75.   It’s a good investment that should last for years.  Some articles recommend putting seedlings on top of the refrigerator for warmth.  I don’t know about you, but I rarely see the top of my refrigerator (which explains the dust piles).  You’re more apt to forget about the seedlings and they die from lack of water.  Heat truly speeds up germination. The bottom heat but cooler air above can give your seedlings sturdiness.

You don’t want spindly plants.  Some seeds, like lettuce and spinach, will do fine without heat.  In fact, spinach senses when conditions are too hot, and it won’t germinate. That’s why it is a Spring and Fall crop. Other crops, like eggplant and peppers, are heat loving and don’t want to go outside to play unless it’s at least 50 degrees at night. You’ll want to water your seedlings so they don’t dry out.  Sometimes that can be daily, if using a heat mat. Once plants are up, they need as much light as possible.  Energy-efficient LED lights can provide overhead lighting without draining your bank account (both in acquisition and use).  However, if you have regular fluorescent lights, they work too.  Most seedlings want 12 – 14 hours minimum to do well.  Some seed packets state “needs light to germinate”.  That means you’ll want to just press the seeds into the starter mix and possibly “dust” a little sand/starter mix on top.  You still want to be able to see the seeds. 

Hollyhocks are a good example of this.  When your seedlings are getting bigger, you should pot them up into a bigger container if the weather outside is still not safe.  They will benefit from daily outings.  Be careful not to overdo it. Think of a Yankee heading to the Florida beach and spending a full day in the sun.  Morning sun is less severe.  Afternoons can burn your plants. Start slowly.  An hour or two and work up to all day. In addition, days can be very windy.  A light wind is good as it strengthens the seedlings’ stems.  Too much wind and your plants will be overwhelmed.  Going outside can suck up moisture quickly.  Water might be needed.   Row covers (white cloth that is sold in varying degrees of protection) can cut both the wind and the harsh sun, once you decide to plant in the garden.

The best time to transplant seedlings is near the end of the day.  They’ll have the evening to recover and wake up to a wonderful new apartment.  Another really good time to plant is during overcast (or light rain) days.  Water your plants well so they can settle in.  If all else fails, early morning works but watering becomes even more critical.  Transplant shock can be fatal!  Although not included in the “Water, Light and Heat” category, it might be helpful to understand that some seeds like to “experience” winter.  If you’re saving seeds, that’s great.  Just know that hybrids (F1’s) are different than OP (open pollinated) seeds.  I always had a hard time getting rosemary seeds to germinate until Johnny’s Seeds offered “primed” seeds.  Now they reliably sprout.

All of this sounds easy until you factor in going off to your day job.  It really can be a full-time job watching your seedlings.  Like children, it can be very rewarding.  And just like children, you’ll want to choose your battles.  Grow what gives you joy, and purchase the seedlings that frustrate you.  Reach out to fellow gardeners.  We love to share our successes, tips and ideas.  Happy gardening.