A Grower's Guide to Sowing Seeds

Table of Contents

With frosted mornings and sun-kissed afternoons, telltale signs of Spring initiate the gardener’s longing to get their hands in the dirt. In the Northern Hemisphere, this begins March 20, which touched my Zone 5 garden with a burst of warmth and light. Shrubs are budding, the ground is thawing, and I am eager to sow seeds.

Alas, by trial and error, my February-sowing has yielded no germination, as seeds will not be hurried. The life fully-encompassed within an oblong shell sprouts in its own time, when temperatures are optimal and moisture is abundant. What early efforts contribute to a successful garden? How does the gardener provide favorable conditions for seed?

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Warmth, Water, and Soil

Though seeds contain within their shells nearly all the plant needs to grow, there are three factors that heavily affect the success of a seed.

Warmth

To germinate, seeds typically require their blanket of dirt to stay around 70 to 80 degrees Fahrenheit, though some cold-weather seeds enjoy a snug 60-75 degrees. With lower temperatures, germination slows down and can take up to five weeks. At optimal temperatures, seeds will sprout within a handful of days and begin their journey of growth.

The following peer-reviewed data is provided by Alabama A&M University and presents a range of germination temperatures for common vegetable seeds.

Seed Germination Temperatures

Vegetable	Minimum (°F)	Optimal Range (°F)	Optimum (°F)	Maximum (°F)
Lettuce	35	40-80	75	85
Onion	35	50–95	75	95
Parsnip	35	50–70	65	85
Spinach	35	45-75	70	85
Beet	40	50–85	85	85
Cabbage	40	45–95	85	100
Carrot	40	45–85	80	95
Cauliflower	40	45–85	80	100
Celery	40	60–70	70	85
Chard, Swiss	40	50–85	85	95
Parsley	40	50–85	75	90
Pea, English	40	40-75	75	85
Radish	40	45–90	85	95
Turnip	40	60–105	85	105
Asparagus	50	60–85	75	95
Corn	50	60–95	95	105
Tomato	50	70–95	85	95
Bean, lima	60	60–85	85	85
Bean, snap	60	65–85	80	95
Cucumber	60	60–95	95	105
Eggplant	60	75–90	85	95
Muskmelon (cantaloupe)	60	75–95	90	100
Okra	60	70–95	95	105
Pepper	60	65–95	85	95
Pumpkin, Squash, Watermlon	60	70-90	90	100
Squash	60	70–95	95	100
Watermelon	60	70–95	95	105

Generated by wpDataTables

Providing Seeds Warmth

A few rules apply when creating a warm environment for the infant seed: light generates heat, soil retains heat, and the amount of heat allowed will affect how hardened off the seedling becomes. I will explain several warming methods I have used to sow seeds.

Cold Frames

In the months of bitter wind chill and frost-laden nights, cold frames offer a protective home for the little seeds. These covered beds can either contain a patch of dirt for direct sowing or a lining of organic matter like hay or grass clippings on which seed trays may rest.

With a south-facing and sloped clear cover, an abundance of light warms up the bed during the day, and the walls protect the seedlings from the worst of the weather and wind at night.

A caution now is given to ensure the beds do not warm too significantly during the day, as temperatures in a hot enclosed bed can reach upward of 100 degrees, even in the dead of winter, just as a car in the sun captures heat. When the sun is clear, vent your frame to allow temperatures to stay within the acceptable range, then close your lid at dusk to retain heat from the day.

A handy thermometer will be an invaluable tool. For cold frames that lack a power source or retain too much moisture, an analog thermometer is best. For the gardener who values technological notifications, a battery-powered digital thermometer that connects to an app on one’s phone is optimal.

Benefits: A cold frame can extend the growing season, as a gardener can plant seeds earlier and later in the year with greater control over the environment. Cold-hardy plants may even grow over Winter in a cold frame. Plants will also be more hardened off to the daily change in temperatures from a cold frame, which means you can directly transplant them into the ground when they reach a more mature size.

Final notes: DIY your own cold frame using the following plans from HGTV, or find inspiration from our Pinterest board “Cold Frames” here. For my own cold frame, I used 1″x6″ cedar boards and Suntuf Polycarbonate Greenhouse panels.

For treated boards, line the edges with hay or grass clippings and plant in trays, yogurt cups, or whatever pot you can find. Ensure good drainage with holes. For non-treated boards, you can directly add dirt and sow into the dirt. The large amount of dirt will retain heat easier, as well as moisture.

Milk Jug Terrariums

This sustainable solution upcycles used milk jugs and works similarly to a cold frame. Translucent thick and durable plastic provides protection from the elements while still admitting light. The container is horizontally bisected, dirt and seeds are added, then the little hut is duct taped until sealed. With the lid removed, water and air circulation allow the contained ecosystem to develop, and when the plants germinate and outgrow their place, they can be directly transplanted to a growing bed, as they are hardened off sufficiently while in the jug.

Benefits: Not only does this method reuse wasted materials, it also allows a gardener to start in colder months, also known as “winter sowing.”

Growing Lamps

When starting seeds indoors, heat and light are just as important as outdoor, but may be less readily available than what is naturally provided. A growing lamp is a two-for-one solution, providing both heat and light, and typically just enough to give your indoor garden the help it needs.

Generally an adequate growing light uses 20 to 40 watts per square foot of coverage, and there are several sources that help gardeners choose the right light for them. In my experience, a red-blue LED light panel is a trustworthy solution, as is a south-facing window sill.

Just remember that the dimmer the light, the closer it will have to be to your plants to provide sufficient light and warmth, and when your plants reach an adequate size (or when the frost has passed), they will require hardening off by taking them outdoors for small intervals throughout the week until they can handle the change in temperature, air, and environment fully.

Benefits: Seeds indoors germinate easier and have a higher chance of survival if hardened off correctly. Plus, seeds started under a grow light can begin their journey at any time in the year. This means certain plants which do not require pollinators such as leafy greens can be grown year-round.

Seed Tray Heating Mat

As an avid fan of Monty Don in his long-running BBC show Gardener’s World, I consistently hear his advice to place your seed trays above your radiator heater, or to use an electric heating mat beneath a tray of seeds. Though I myself have not employed this method, I find that I may in the future, given the reluctance of my seeds to sprout at the end of winter this year.

While space is a concern when starting seeds indoors, a variety of options are available to fit a gardener’s spatial needs.

Benefits: This method ensures adequate heat will reach your seeds, as the root area will be consistently warmed above ambient room temperature.

Water

Water is fuel that flows through all living things. It cannot be understated in the garden, but too much water promotes rot, fungi, and disease. As with all things, moisture necessitates balance and understanding a seed’s needs.

When starting your seeds, ensure they are watered adequately. It is the consistent moisture, as well as a warm environment, that signals the seed to sprout. Water will break down the seeds’ shells, which provide additional nutrients to jumpstart root growth. The soil should be moist to the touch, but newly-added water should easily permeate the dirt. If water is pooling up instead of soaking down, this is a sign that the new seedling is being overwatered.

As important as water is to life, air is equally important to health. Air promotes healing and growth, reduces bacteria and viruses, and provides particles of much-needed oxygen to roots. A boggy pot or plot stifles the air and asphyxiates the seedling. Additionally, a heavy flow of water will wash out the seed, destroying any roots that were diligently growing.

Watering Indoor Seeds

A few methods exist to provide the desired 85-95% humidity level for germinating seeds indoors:

Soak new seeds in water before planting. I have had success in using dampened sturdy paper towels, then placing my seeds between the layers and leaving them in a plastic zipper bag until they sprout. Alternatively, allow the seeds to soak for about ten minutes in a tray of water, then add soil to the tray an allow the seeds to germinate from where they lay. This will give them that first signal to sprout.
Cover the watered seed trays or containers in plastic until the seed begins to germinate. As little water will be lost to evaporation, the plastic can remain until signs of life emerge from the soil.
Spritz water regularly to keep the soil damp. This will prevent the seed from washing out of the soil or overwatering. If the soil dries throughout the day, spray the top of the soil a bit more.
Dip seed trays or containers in water to promote root growth, but ensure proper drainage such that root rot will not occur.

Watering Outdoor Seeds:

Remember that greater amounts of soil will result in a longer time between watering. If your pot is small, water it frequently, at least daily. If you are direct-sowing seeds in a plot of land, natural rainfall may be enough in the Spring to encourage growth. A gardener may be tempted to mulch their soil to help it retain moisture, but take caution that mulched material is not too difficult to break through, as tiny seedlings have little strength.

Soil

Much can be stated about the growing medium of seeds, but three requirements are necessary of soil for successful seed germination: water retention, loose texture, and sufficient nutrients.

Water Retention

In the section above, I wrote about the need for consistent moisture. Soil with increased water retention helps by creating an environment where water does not drain directly away from the seed, which lacks long-reaching roots. Peat moss or coconut coir, and vermiculite or perlite provide an excellent growing medium for water retention and aeration, respectively. General potting mix usually contains perlite or vermiculite sufficiently, as well.

Alternatively, hydroponics allows a gardener complete control over the environment, as roots develop directly in water and soak up added nutrients there. Though I have little experience with this method, I am eager to discover its best practices for myself.

Loose Texture

In my garden, the majority of my natural soil, below the layer of top soil, is clay. The first year I attempted to grow carrots, I sowed directly into the ground. The result were bright orange nubs, no longer than my thumb. Delicious as they were, I learned how a loose soil is important in gardening, and even more so for germination.

When a seed pod explodes with life, the organic shell is pushed aside as roots penetrate deep and the stem sprouts upward. In a compact soil, there is no room for the roots to develop, which in turn provides no support for upward growth. Alternatively, roots may grow crossed or spiraled, promoting rot and disease.

With germinating seeds, a lightweight soil of finer particles promotes healthy root growth. This includes vermiculite or perlite, which are both lightweight, though vermiculite allows for greater water retention. While germinating soil is incredibly easy to mix for oneself, I have trusted the FoxFarm brand over the years.

Sufficient Nutrients

For the first few days of its life, a seed has packed all that it needs to get a head start into the world. However, once roots develop and leaves unfold to collect sunlight, soil may need to be amended to ensure the budding seedling receives enough nutrients. When a seedling has reached this stage in its life, transfer it from its germinating soil into a potting soil rich with the right nutrients.

One easy and free way to provide your new plants with these supplements is to soak kitchen scraps like banana or fruit peels in a container of water, set aside water used to rinse rice, or collect water from a fish tank. Use the water from each of these sources to feed your seedlings or even larger and established plants. Additionally, seaweed extract serves as an excellent organic plant fertilizer.

Whether your garden begins within the walls of your home, in a sturdy cold frame, or out and directly in the rich soil of our Earth, each effort to start new life will bring a gardener deep sensibility, self-reliance, and satisfaction. Continue to join me at Nectar and Nettle as we begin our journey with a seed.

A Grower’s Guide to Sowing Seeds