Growing Healthy Seedlings

Seeds are alive. I think we sometimes forget this when we buy seeds in a package from a store shelf, or stick them in a drawer to search for next year. But seeds are living beings just waiting for the right conditions to grow. They have all the energy and nutrients stored inside to become small seedlings with their first leaves (called cotyledons). To enable the seed to store for a long time and stay alive (called viable in the seed industry), you need to store them in a way that reduces the seed’s need to consume its stored energy. So, keep them somewhere cool, dry, and dark.

When we want that seed to grow, we use the opposite conditions: warm, humid and bright. The most reliable way to control those conditions is by starting seeds indoors. We sow seeds both indoors and directly out in the field. It depends on the crop, the time of year, and the weather or field conditions.

Here I’ll describe the conditions that support the growth of seedlings and the ways we meet those conditions indoors on our farm.

Propagation is something I love and, even though our set-up is fairly simple, we’ve had good success. We grow all our own seedlings (from seed and cuttings), as well as seedlings that we sell through our spring plant sales (in Dieppe, NB and at the farm).

Heat for Seedlings

Plants may have different temperature requirements throughout the different stages of their life. For example, the majority of the seedlings we grow in the early spring are cold-tolerant plants. Things like lettuce, bok choi, calendula, larkspur. They can be started outdoors, but early in the season, we start them indoors so that they will germinate and grow to a transplantable size faster (and likely with a higher degree of success) than if we’d direct-sown them in the cold or snow-covered soil in the field. But once grown large enough, these plants that we started with heat will be transplanted out to cool soil and varying weather conditions. We will likely move them to an unheated bench midway through germinating and transplanting since they no longer need the heat.

Even with a hot weather seedling like tomato, it benefits from warmer temperatures in the seed germinating stage compared to the mature pre-transplanting stage.

Some seed catalogues list ideal germination temperatures for each crop (like High Mowing for vegetables). You can also look on the internet. Fedco Seeds has great charts for vegetables, herbs, and flowers.

A seed can germinate below its ideal temperature but will likely take longer. Without the ability to achieve ideal temperatures, you may need to start your seeds much earlier than another grower.

Ways to Add Heat

To add heat, there are 2 general methods – air heat or bottom heat. In either case, the temperature is ideally maintained with the help of a thermostat.

Heating the air can include the heat in your home. Heating the air in a propagation greenhouse might include a furnace or wood stove.

In this case, for insulation purposes, growers typically use 2 exterior layers of plastic on the greenhouse, inflated with air between them.

Bottom heat means heating the “root zone” under the plant. This could come from heat mats or greenhouse tables (or benches) outfitted with electrical cables or hot water pipes. Some growers direct the heat from their furnace or wood stove using forced air through large poly tubes running under the seedling benches.

Moisture for Seedlings

The moisture level can be one of the hardest to manage when growing seedlings. It will depend on the temperature and light levels (which affect evaporation), the stage of life of your seedling, the air flow or lack if it, the size and type of pot or plug, and the type of potting soil you use.

Some people “bottom water” (which means putting the tray on top of a shallow tray of water to wick up or absorb into the potting mix).  Others water from above, with for example a sprayer or hose and fine nozzle, or watering can.

On our farm, we primarily top water. Though in our house, for houseplants, we often bottom water because it’s less messy.

If, for example, our seedlings did not get much in the way of air flow, bottom watering could be helpful to reduce or eliminate leaves that stayed wet for a long time and be more prone to disease.

Air Flow

 We prioritize air flow for our seedlings, both for moisture management and to get the plants used to wind from an early age. We let the wind pass through the propagation greenhouse with lots of ventilation as much as possible. Plenty of air flow from an early age also helps prepare plants for transplant. “Transplant shock” comes about when plants experience very different conditions once planted out in the field like wind, bright sunlight, and less soil moisture.

Disease Prevention

Diseases, including damping off, can relate to moisture. We rarely have those issues on our farm and focus on preventative strategies. We keep a strict eye on moisture in the greenhouse (our goal is no dripping from the roof, ever). Good air flow with fans, vents and opening up the greenhouse. And we use an organic soil-based potting mix. Many non-organic growers choose soil-less mixes for the same reason. But I believe that healthy soil is the basis for disease prevention, and, at least on our farm, that’s worked out for us.

Covering Seeds

We cover our seeds with a light dusting of vermiculite rather than potting mix. The reason for covering seeds is to prevent them from drying out as they’re germinating. Vermiculite can do this and it’s easier to spread on a thinner layer of it, though potting mix is often used and doesn’t require buying another product. If a seed has less “vigour” (vigour refers to the energy the seed has in it to grow, seeds tend to lose their vigour as they age), it may not have the strength to grow up through a thicker layer of soil or vermiculite). This could be a reason for lower germination numbers.

Light for Seedlings

Not all seeds need light to germinate (most vegetables don’t) but as soon as a green tip appears above the soil, they need light. Without enough light, your plants will get “leggy” (when they reach for more light and get spindly). With more heat, the young plant will also expect more light (that’s generally what they’d expect outside). So, a heat mat next to a sunny window may be fine for germinating, but often won’t provide enough light to prevent leggy-ness of the plants as they grow. In that case, removing the trays from the heat mats after germination could help (also looking for a sunnier spot or adding grow lights).

Creating a Custom Propagation Space

You are unlikely to create or have the exact same set-up as us or anyone else. In fact, on the over a dozen farms I worked at before starting our farm, none had the exact set-up, and many had extremely different propagation spaces. Some used grow lights on shelving units indoors, others in greenhouses with furnaces or wood stoves, others with bottom heat with hot water pipes or electrical cables.

I’ll describe what we do currently and first mention a couple of things we have done that we no longer do.

The Early Years

In our very first year on our land, we didn’t have any infrastructure. We started seedlings on our dining room table, on a long bench we built in our south-facing indoor porch (with electric heat underneath), on a fresh-horse-manure-heated outdoor bench, in an unheated caterpillar tunnel (with lots of row cover piled over at night), and finally, once it was up, on the heated bench we made in our high tunnel.

Over the years we added heated benches to another tunnel and even outside (with 1 layer of plastic over the bench) and only last year did we move all our heated and unheated benches to the same structure (for our own convenience).

Our Current Set-Up

We grow the majority of our seedlings in our propagation greenhouse.

Bottom Heat

We only heat the benches, not the air. Our bottom heat comes from electrical cables. There are insulted panels under the cables to prevent heat loss. Each table has its own thermostat and is programmed to turn off or on at certain temperatures. Our tables are set at slightly different temperatures, for different types of plants. Besides miserable cold days, the heat is often off during the day and mostly on at night.

Greenhouse Plastic

The exterior of our greenhouse has a single layer of plastic and we use another layer of plastic over hoops above each bench. We prefer a single layer on the outside for increased sunlight (every layer of plastic or row cover reduces light transmission). On cold nights, we add row cover over the bench’s plastic.

Venting and Covering

Every day, we remove any row cover and pull up the plastic over the benches to maximize the amount of light reaching the seedlings and for air flow. If it’s warm enough outside, the various doors or window will be opened. We keep the vents open most of the time, unless it’s extremely cold because we don’t like excess moisture and don’t want to see any dripping from the ceiling. If it’s too cold outside, we turn a fan on.


A big downside in relying on electricity is the potential of losing power. One year, we lost power in a snowstorm in April. Besides bringing some of our most sensitive seedlings (basil, peppers) into our house, we added old wool blankets over row cover and plastic on the benches as extra insulation overnight.

Rolling Benches

Last year, we built a new greenhouse for propagation. It’s a smaller space than we used previously, so we needed to maximize our bench space. We decided to build rolling benches. The tops of the benches roll sideways, into the pathway space, so at any moment, there’s only be one path available to walk down in the greenhouse. These types of benches are used most often in the nursery and cannabis industries and are expensive to buy. We couldn’t find any plans online to DIY them, so we just winged it. They certainly work the way we wanted them too, though as in so many projects, the last bench we made is better than the first one.

Pest Control

One issue that many growers have in propagation greenhouses is rodents (seeds are rodent food!). The cats who live with us have always helped with that. I know some growers who have lost thousands of dollars worth of seedlings to rodents, even with a cat. Just like people, not all cats are interested in the same kind of activities, I suppose. But our cats have made any losses too minimal to recall.

I won’t go into great detail here, but we also use a few other spaces to start seeds indoors.

The very earliest seeds we start, including onions, leeks, certain flowers like lisianthus, ranunculus, and anemone just get started in our house. We do this because we don’t want to start up the heated benches in the propagation house just yet.

Germination Chamber

We also use a germination chamber we DIYed (a whole blog post on our website is devoted to the construction). It’s a humid, heated environment. It’s also dark so we need to get seedlings out as soon as they pop up.  A benefit is that this is hands-down the fastest way that our seeds germinate. Because it’s a very humid environment, we don’t need to water the trays at all while they’re inside.

While I’ve mentioned many aspects, I’ve really only skimmed over each topic and didn’t include any details of other aspects of our propagation work (like potting mix choices and fertility management). My hope is that you’ve noticed a few areas where you might be able to better meet the needs of your seedlings that will work for you.

This article was written by farmer Shannon for Rural Delivery Magazine, a great magazine about rural life in Atlantic Canada. We highly recommend you subscribe!

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