How to Start Seeds Indoors During the Winter

Tools and tips for starting seeds indoors including when to sow seeds, grow lights, propagators, and ways to successfully grow seedlings during the winter.

Gardeners with mild winters and warmth early in the spring have it easy. You can practically throw seeds in the soil outside and have a harvest in no time. Others sit watching the snow pile up, feeling impatient to sow seeds too. If you’re in that situation or have cooler temperatures in spring, starting seeds indoors is a way to get growing long before you could outside in the garden. It’s also helpful for those who have short summers since and want to grow crops that need a long time to mature.

This piece focuses on sowing seeds undercover to get a head start on spring. It’s a smart move that lets you bypass weeks and months of cold and gives you plants that are well ahead of those grown outdoors. We’ll go through the best time for starting seeds indoors, grow lights, plant propagators, and other ways to grow tender seedlings even while it’s freezing outside.

Seeds to Sow in Winter

When you start seeds indoors, you control light, moisture levels, and temperature with various tools and methods. Most set-ups require growing plants indoors or in a heated area. Done in the right way, you can plan to have healthy plants ready for planting when it’s safe to put them outside. Growing undercover can save weeks on your first harvests and flowers and allow you to get an extra crop in for your gardening zone. It works for most vegetables, from frost-tender peppers to leafy greens like spinach. Crops that you can start from seed in the winter include:

• Eggplant (aubergine)
• Chilis
• Peppers
• Tomatoes
• Onion seeds
• Even more can be found here

Early Seed Sowing at a Glance

When sowing seeds in winter, there are some tricks of the trade that will ensure success. Most seeds need warmth and bright light to begin to germinate and grow. That’s why grow lights and other specialist gardening equipment are necessary if you wish to start seeds early. In the above video I give you an easy-to-understand introduction to sowing seeds in winter and starting them off undercover. Here are the most important points:

• Sow seeds based on your last frost date and time it takes the seedling to grow
• Start seeds in a place that doesn’t dip below 7C/45F
• Grow seeds in individual pots, modules, or seed trays
• Use a seed compost rather than potting compost or garden soil
• Seedlings need light, moisture, and warmth to grow
• Use grow lights, propagators, heat mats, and greenhouses to start seeds early
• Harden plants off and plant out after your region’s last frost

The Best Time for Starting Seeds Indoors

When sowing seeds outdoors, you wait until the soil’s temperature reaches at least 45°F (7°C) since most seeds won’t germinate below that. Ideal seed sowing temperatures for edible crops vary, but a good rule to work by is that heat-loving plants, like tomatoes, germinate between 60-86°F (16-30°C), and temperate veggies such as lettuce, beets, and onions need at least 41-59°F (5-15°C) to sprout but prefer it a little warmer though. These are conservative and safe temperatures, but individual plants’ temperatures will be more specific. Though you can make your own free-draining seedling potting mix, use sterile seedling compost for best results.

The time these optimal temperatures occur in the soil outdoors will be different based on your region and gardening zone. For the luckier gardeners, it comes a lot earlier than others and can save time and effort. Although more involved, growing undercover allows gardeners to sow seeds earlier in the year and get a head start on spring.

Grow Seeds Undercover

Starting seeds undercover gives them a protected environment that mimics warm spring to summer conditions. There are several ways to do this, but whatever method you choose, you must provide your seeds and seedlings with a place to grow that’s warm, light, and protected from the elements.

A temperature of 18-24C (65-75F) and bright overhead light are optimal. Seedlings also need a humidity of 50-70%, so you’ll need to monitor this as well. A gentle electric fan is also a good idea for starting seeds indoors. They can cool seedlings down and also mimic the movement of the wind. This can make the seedlings grow stronger, and make them more resilient when planted outdoors.

You also need to provide your seeds and young plants with adequate drainage, the right moisture level, a suitable growing medium, and protection from pests and pets. There are many ways to go about it, and further below, we’ll discuss the most popular ways to start seeds indoors.

Growing Seeds Indoors based on Last Frost Date

You cannot always trust the sowing information given on the backs of seed packets, especially when it comes to sowing seeds early. Seed packets are sent all over the country, and April in Maine has different temperature and light levels than April in Texas. How you determine when to sow your seeds comes down to your specific last frost date. That’s the date you can safely know that young plants won’t perish if you plant them outside.

In another article, I go through the earliest seeds to sow and when. The main idea is to start your first seeds based on how long a particular type of vegetable needs to grow before you can plant it in the garden. The timing also relies on your garden’s specific last frost date and outdoor temperatures.

If you plant out before the last frost date, you can lose your crop to a freeze. If you sow plants way earlier than you should do, and they’re ready to go outside before their last frost date, they can become weak, leggy, or pot-bound while they wait inside.

Useful Tools for Starting Seeds Indoors

To start seeds indoors, or undercover, you need the right tools. It can be a little confusing as to what each does, why we need it, and how it can help us start healthy seedlings. Below are the main tools that the typical gardener uses to grow young plants early in the year. Depending on where you’re growing your seedlings, you’ll need one or more of these:

• Propagators, for humidity and warmth
• Heat mats, for warmth
• Grow lights, for light
• Greenhouses, for light, warmth, and humidity

Starting Seeds in a Propagator

Plant propagators come in many forms, but with the basic premise of creating a mini-greenhouse around the seeds or plants. This enclosure keeps warmth and moisture levels in and may also have a vent on the side to release the excess. Propagators are a useful tool for starting seeds indoors since the environment inside will be more humid than the air of a typical home. You can also use them in greenhouses early on in the year.

The most common type of propagator I see is a plastic seed tray with a clear domed plastic lid. It can be placed on a warm, well-lit windowsill, an indoor grow light set-up, or a greenhouse bench to provide an incubation environment for seedlings. Simple propagators are often used in spring when light and warmth levels are decent in the greenhouse or a well-lit room such as a conservatory.

Heated Propagators & Heat Mats

A step up from this is a propagator that’s heated from below. Though some people use a heat mat under their seed trays, I have an electrically heated propagator. It looks like a long plastic trough with an electrical cord, and three small trays can sit inside, each with a clear lid. It doesn’t have a thermostat, but higher-end propagators do so that you can set it at the exact temperature you need.

Gentle bottom-heat speeds up germination and can help you grow seedlings in a cold room, garage, or basement. You can also find heated propagators that can cover several seed trays and even have built-in grow lights. These are useful if you want to start seeds earlier in the season or your growing area is dim.

Starting Seeds Indoors with Grow Lights

Grow lights are artificial lights that you use to give plants the light they need to photosynthesize and live. They are also essential if you plan to grow plants from seed earlier in the year when the natural light levels are lower. You can purchase custom-made grow lights from specialty suppliers or create your own set-up. No matter how you do it, the most important thing to know about grow lights is that there are three main types. Each has its pros and cons in regards to light levels, cost, and energy usage.

Fluorescent lights (CFLs) are the least expensive and most accessible. Though technically you could use any fluorescent light (and yes, even the ordinary ones), traditional-shaped bulbs placed in a lamp give less light coverage than a tube light. Tube fluorescent lights (T5 models being the most popular) placed in hanging lamps with adjustable height levels allow you to grow more plants.

You have the choice of using standard fluorescent bulbs if you like, and many use them to great success. There are also specialty grow light fluorescent bulbs and if you opt for them, use cool light bulbs in the 6000-6500K color temperature. That information should be available on the item description or packaging. They’ll also be described as ‘cool light’ or ‘daylight’ bulbs. Grow light bulbs also come in warm light, and these are used by growers who want their plants to flower under lights. More specifically, cannabis growers.

The other two types of grow lights are compact fluorescent lights (CFLs) and light-emitting diodes (LEDs). CFLs are similar to standard fluorescents, but they’re much more energy-efficient. LED grow lights are more expensive to buy, but they’re the most cost and energy-efficient for the hobby gardener in the long run. LEDs often have colored lights to recreate the spectrum that plants need to grow.

Avoiding Leggy Seedlings

If you’ve ever started seeds and have watched them grow tall spindly stems, then you know what leggy seedlings look like. The main reason is that a lot of people try starting seeds indoors on their windowsills, where there’s insufficient, one-directional light. Windowsills don’t usually provide enough light for seedlings and cause them to grow ‘leggy.’ It can also be caused by placing grow lights too far above the seedlings. With cool fluorescent bulbs, you tend to hover them just 2-4″ above the seedlings’ canopy.

To the beginner, leggy growth may look as if the plant is growing strong. However, tall thin stems mean the plant is struggling to get enough light and trying to grow closer to the sun. Leggy growth can also be caused by the seedling being too warm. This can happen if you leave the seedlings growing above a radiator or on a heated propagation set-up for too long.

In most cases, leggy growth leads to stunted growth and weakened stems. They will struggle when you eventually move them outside to the garden. Plants that start their life leggy are also more susceptible to pests and diseases and create more work and a smaller harvest. If you don’t have space or the funds to install a grow light set-up, you can get inexpensive grow lights that clip onto your windowsill. That way your seedlings get light from the window and from above.

Starting Seeds in a Greenhouse

Greenhouses are a gardener’s best friend, but they don’t come cheap. If you’re fortunate enough to have a greenhouse you can use it to start seeds earlier than you would be able to outdoors. Unheated, you can sow seeds that germinate at lower temperatures such as onions and lettuces, but ideally, it needs to be at least 45-59°F (7-15°C) inside day and night.

If there’s a way to heat your greenhouse or add heat mats under seed trays, then you have an amazing propagation area with plenty of light and warmth. Temperature and humidity levels in a greenhouse are easy to monitor with a humidity sensor.

A tip for keeping greenhouses warmer at night is to place a large black plastic bin filled with water inside. During the day it will gather warmth from the sun, and it will slowly release that heat overnight. You can use the same method in cold-frames by filling large plastic drink containers with water.

Though you might be tempted to use a small clear-vinyl “greenhouse”, these aren’t ideal for starting seeds outdoors before spring. It’s just too cold, and they don’t retain much heat. You can use them indoors though, or in a place that’s heated, if enough light is also provided for young plants. The plastic cover that zips over them keeps in warmth and moisture in the same way as a propagator does.

Seedling Stages

Sow seed into sterile seed compost and after 5-14 days, tiny spouts will emerge. These little green stalks will become this year’s crops and, depending on the type, will need 2-10 weeks to get to an adequate size to plant out. Those initial seed leaves will give way to true leaves in the next stage and continue filling out over time. Your seedling is ready to be planted in the garden when it has three to four true leaves.

If you’re using grow lights, ensure that they hover at the right height above the canopy of your seedlings. This is why having adjustable grow lights are crucial. Have the lights too close, and the leaves can scorch. Too far away, and the seedlings will grow leggy and weak. The type of grow light you’re using and its wattage will determine this distance but as mentioned previously, it’s usually only a few inches above. If you feel a lot of heat coming off your bulbs, keep them at a safe distance from your seedlings.

Repotting Seedlings

In between the time you sow seeds and plant them out, you may need to re-pot seedlings. If you start seeds in a tray as I did with these tomatoes, you will need to re-pot the tiny seedlings into individual modules or pots after they have two true leaves. If you don’t do this, the plants will quickly outgrow their tray and overwhelm each other. When you re-pot seedlings, use fresh potting mix and upgrade to a type suitable for young plants. It will have more nutrition than seedling compost, which is only suitable for the first few weeks when a seed grows into a small plant.

Of course, you can start your seeds in pots that are big enough to hold a mature seedling. The benefit is that you won’t need to re-pot the seedling on, and in this case, you should use standard multipurpose compost. The downside is that you can lose grow light space if you do this. Imagine the space that seedlings would need you could start in small modules (or trays) versus larger pots. Then how many grow lights you’d need to cover that space. Since some plants need more time growing undercover than others, it’s all a juggling game.

Choosing the right pot or module to pot seedlings on will depend on the type of plant you’re growing. One way to work this out is to imagine the vegetable plants for sale at your local garden center. Young tomato plants often come in 3” modules or pots, and young lettuce plants are usually in modules sized 1-2”. Have a look around to see what size and shape containers other veg is growing in for a better idea. Also, keep in mind that you sometimes won’t have to buy new either – recycled seed starting containers are a great way to save money and reduce waste.

Hardening off Seedlings Grown Indoors

Hardening off refers to gradually getting your seedlings ready for the great outdoors. While you’ve been carefully growing them undercover, they’ve become used to steady and controlled conditions. It will also likely be a lot warmer than what it is outside.

To get them prepared, you expose them to outside conditions during the day and protect them at night. For plants grown indoors undercover, you harden them off for three weeks before you plant them in the ground. You only expose your plants to outdoor conditions when it’s not freezing, and conditions are calm and relatively dry. You also begin hardening off after your garden’s last frost date.

Hardening off Seedlings with a Cold-Frame

The easiest way to do this is with a cold frame. Cold-frames are like small and short greenhouses, except only their slanted roof is made of glass (or another transparent material). The other four sides can be wood, plastic, straw, or brick. In colder temperatures, you really need the walls to be a material that will insulate the plants at night.

During the day, you leave plants inside the cold frame with the lid open. At night, you close the glass lid to protect them from the night. You can also leave the top down on colder or windier days. I think you’ll agree that it’s a lot more convenient than taking seedlings outside and back in every day. After the three weeks are up, you can safely plant them out in the garden. Planting them out earlier can result in shock, and the plants can die or fail to thrive. It also makes sense to also take these precautions to protect spring crops from the cold.

More Seasonal Gardening Inspiration

For even more tips on how to grow plants from seed, check out these pieces:

• A list of the first seeds you can sow
• Tips for planting bare-root roses
• Tips for starting a new vegetable garden
• Grow Cut-and-Come-Again Lettuce and Baby Salad Greens