Get a head start on the growing season and enjoy an early harvest and greater yields by starting seeds indoors. In colder climates with shorter growing seasons, starting seeds indoors gives slow-growing crops like tomatoes and peppers a head start to reach maturity. In warmer climates, this practice gives the opportunity to get an extra round of crops before the heat of summer stifles growth. Another perk is the monetary savings. Purchasing seeds is more cost-effective than buying established plants from a nursery.

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SUPPLIES QUICK LIST

*I use all of these products except for the grow rack (because I use a grow tent).

CLEAN SUPPLIES

Seed starting has its own phases of needs. From the germination period to graduating into the natural world, seedlings depend on the elements to survive and thrive. When we start seeds indoors it’s important to create an environment similar to the outside world.

Germination Phase

DARKNESS

Just as we emerge from the darkness and safety of the womb, seedlings emerge from the darkness and safety of soil. The contents of this dark environment are important to the success of the seed’s germination. Seed starting mix is a light, sterile, nutrient-less medium that allows seedlings to emerge at ease and expand their tiny roots.

WATER

Each plant will have different needs at different times so it’s suggested to keep each varietal to one tray. OR you can go about it like me and cut sections out to maximize space (see my photo under the LIGHT section to get an idea of what I mean). You can also purchase smaller cell trays like these: 10 Cell Trays or 6 Cell Trays.

WARMTH

Seedlings need warmth to speed up the chemical reactions that take place in the seed. Warmth also speeds up the making of new cells when the plant embryo is growing.

Most seeds germinate when the soil temperature is between 70°-85°F. Use a dial thermometer to check the soil temperature regularly.

Warming mats are used to gently warm the soil, promoting germination. They aren’t 100% necessary but they do facilitate faster germination. If you choose to use a grow tent you can achieve the same results and forego warming mats by relying on warmth generated from a heater. If you use warming mats you most likely won’t need to use a heater until the seeds sprout.

Once you see the first sprout it’s important to remove the warming mats immediately. Leaving mats on may spur rapid, lanky, weak growth or encourage fungal diseases at the soil level for seedlings past the germination phase.

AIR

Another vital necessity for seed germination is oxygen. This allows seeds to produce energy which takes place when the embryo breaks down its food store. This process is known as aerobic respiration – a series of reactions where energy is released from glucose, using oxygen.

Once a seed sprouts remove the cover immediately so air can circulate around seedlings, minimizing disease problems and allowing the plant to breathe.

Seedling Phase

LIGHT

Once a sprout is visible it’s crucial to get it under the light. Unfortunately, a south-facing sunny window alone won’t provide enough light for starting seeds indoors. There are a couple of things to keep in mind when using artificial lighting:

To prevent leggy seedlings make sure the distance from the top of the plant and light source is not too far but not too close. Too far and the seedlings will stretch trying to reach the light causing weak stems. Too close and the light can burn the plant. Instructions from your lighting supplier or a simple google search can help determine the amount of space needed.

Seedlings need 14-16 hours of light during the daytime, followed by 8-10 hours of darkness. A timer can make managing daylight and darkness easy. 

Choosing a grow light can be overwhelming but it doesn’t have to be. LED lights are generally more energy-efficient, cost more upfront, and emit more lumens. Some gardeners will still recommend T5 fluorescent lights for plants.

I’d say choose lights based on your growing space. It’s typically recommended to suspend LED lights significantly higher above plants than fluorescent lights, making them good for grow tents. T5 fluorescent lights can be kept only inches above plants making them more ideal for grow racks.

WARMTH

Now that the seed has sprouted it’s time to revisit warmth. Once germination occurs, the optimum growing temperature for the seedling is about 10° cooler than the optimum germination temperature.

Since the warming mats are no longer useful, I use a heater set outside the grow tent to manage the temperature inside. Maintain the optimal growing temperature until transplant day.

This digital thermometer is also great for keeping an eye on humidity levels. It’s important to lower the humidity level to 30-50% once they are past germination. Any humidity level over 60% could potentially damage the plant.

I had a mold problem one year after I neglected to create proper airflow and left the temperature too high. The humidity ended up surpassing 70%, yikes!

WATER

Once seedlings have sprouted, to encourage strong roots practice the bottom watering technique. Rather than watering from atop, water is poured into the bottom cell tray and the moisture wicks up through the drainage holes to growing roots. It wets the soil mass completely without dry pockets. After 30 minutes to an hour dump the remaining water so the cells don’t sit in it. A good indicator that the soil had its fill is when you start to see the water surface on top of the soil.

It’s recommended to bottom water in the morning or in the evening. The best water is either rainwater (*best* full of minerals and slightly acidic) or distilled water (the purest form).

This watering can with a bottlenose is perfect for for this practice.

WIND

To prevent disease and weak stems a fan acts as the wind does outside. The wind is a protector and strength-giver for seedlings. It circulates the air which inhibits mold from forming or spreading by moderating the temperature and humidity. The wind also encourages strong, sturdy stems – the foundation for an abundant harvest of flowers or fruit.

Not too long after sprouting set up a fan. Leave it on throughout the day to create a gentle breeze. The fan should be used up until hardening off beings or transplant day.

Vegetative
Growth Phase

NUTRIENTS

SPACE

Nothing hurts more than snipping perfectly healthy seedlings, but it has to be done. When seedlings are a little taller and established thinning is a must so the seedlings are not overcrowded and competing for water and nutrients. I like this fine pruning snips tool for thinning seedlings. Alternatively, you can just pinch the stem at the base. You do not want to pull seedlings out of the soil because this can damage the delicate roots of the seedlings you want to keep.

Eventually, many of your seedlings will need to move out of their cells and into a roomier container. Potting up describes the process of re-planting a seedling into a larger pot.

Trust your intuition when deciding when to pot up. If you think it’s time for a larger space you’re probably right. Does the plant look happy and healthy? How big is it? Has it slowed or has it been growing steadily?

Another good indicator is when the roots are just poking through the drainage holes at the bottom of the cell tray. If you wait too long though, the roots can become root-bound making the plant less likely to flourish.

It’s also important to pot up because as plant roots grow longer they drink more water and dry out more quickly.

Potting up also feeds the plants. We began with a seed-starting mix but now we are planting them in richer, heartier potting soil with more nutrients.

Transplant Phase

SUN

For a short period of time, a seedling needs to harden off to prepare for the outside world. It’s a process that involves gently weaning seedlings from their indoor environment and acclimating them to the harsh reality of the outdoors where they’ll experience the sun, wind, and weather.

Harden off plants two weeks prior to transplanting. Start in small increments. The first couple days leave the seedlings outside for 1-2 hours in the evening or morning when the sun is less brash, or in a shaded area. Increase time spent outside over the course of two weeks. Eventually, they will be able to spend all day outdoors making them ready for transplant.

EARTH

It’s finally time to integrate the seedlings into the natural world. Make sure your plants have at least 3 or 4 true leaves before you consider transplanting. Temperatures should be consistently warm and out of danger of frost.

To ease seedlings into the Earth spray a seaweed fertilizer on the roots to prevent transplant shock.

It’s also recommended to perform transplanting during the evening or on a cloudy day. This way, plants have a chance to settle in without being in direct sunlight.

Some plants like tomatoes can be buried deep (2/3 their height) because they have tiny bumps on the stems called adventitious roots. This means roots form on the upper part of the plant (the stem, leaves, branches) instead of the main root ball allowing them to grow a deeper and more extensive root system. This gives plants an increased ability to take in water and nutrients, resulting in a healthier plant that is less susceptible to drought, disease, and pests. This technique also allows them to have a sturdy structure – perfect for supporting hefty plants. 

FAQ

My seedlings are seemingly keeling over. They look like they were bent over at the soil line. What should I do?

This is most likely caused by damping off. It is a soil-borne fungus, caused by several different pathogens, that attack and kill seeds or seedlings before or after they germinate. It is most prevalent in wet and cool conditions. Unfortunately, there is not a cure, and the seeds won’t resprout. Start over with clean containers and sterile seed-starting mix. Allow the soil surface to dry out slightly between waterings, and place a fan in the room for good air circulation.

Fuzzy white growth or slimy green patches are forming on the surface of the soil, what should I do?

It’s most likely mold and algae growth. Although generally harmless to the seedling, they both indicate that the seed-starting mix is too wet. Allow it to dry out slightly between waterings and set up a fan to gently increase air circulation.

If white mold is growing on the plant, it’s most likely due to too much humidity. Create proper airflow and make sure the growing space is not too hot. Take action before it spreads and kills the plant. My mold issue got so bad I resorted to this fungicide which remedied the situation and killed the mold however some plants were so far gone it was too late. Below is a photo of the aftermath of a plant that didn’t survive the mold outbreak:

My seedlings have pale, stretched-out, weak stems. What should I do?

These are referred to as “leggy” seedlings often the result of inadequate light, both in intensity and duration. Leggy seedlings have weak stretched-out stems and typically are flopping over because they cannot hold themselves up. Make sure seedlings are getting 14-16 hours of light per day along with placing the light source at the appropriate height from the seedlings. Leggy plants can also result from too warm temperatures and overcrowding.

My seedlings have discolored leaves, what should I do?

If seedling foliage is pale green, yellowish-green, or has a purple cast to it, the seedlings are most likely in need of a nutrient boost. Begin lightly fertilizing seedlings when they have a couple of sets of true leaves. Use a diluted (half-strength) mixture for the first few weeks (follow the brand’s instructions). 

My seedlings have pale/white leaves, what should I do?

This is most likely the result of “sunburn.” Make sure your light source is the appropriate distance from the canopy of the plants.

My seedling’s leaves are curling, what should I do?

It could be because of too little or too much water – use a moisture meter to gauge water needs. It could also be a lack of nutrients or too much heat.

My seedling’s leaves are soft, turning yellow, and fading to pale green what should I do?

These symptoms together mean that your plant is overwatered. You could either repot the plant to remove soaked soil or let the soil dry out and water less. Usually, lower leaves drop first, although the whole plant may be affected.

A seed sprouted but none of the other seeds in the cell tray have sprouted, should I wait to turn off the warming mats?

You’ll want to get the tray under the light and take the greenhouse cover off as soon as you see the first sprout regardless of the status of the other cells of that variety. Those other seeds will continue to germinate even if you remove the warming mat and humidity dome.

Happy sowing!

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