Complete Guide to Seed Starting Indoors

Nothing brightens late winter and early spring days like tomato, pepper or cabbage seedlings emerging from potting soil indoors. For many gardeners, seed starting indoors is the official kick-off for gardening season. Get answers to your seed starting questions from the gardeners at Gurney’s – we’re here to help! 

What’s covered in our guide:  

• Essential Seed Starting Supplies 
• Step-by-Step Instructions for Starting Seeds Indoors 
• From Thinning to Transplanting: What to Do After Your Indoor Seedlings Sprout 
• The Best Seeds to Start Indoors  
• When to Start Seeds Indoors 

Essential Seed Starting Supplies

For growing seeds indoors, you need a few essential materials: 

• Containers or pots, such as a plug tray or peat pots 
• Drip tray, such as a propagation tray or baking sheet 
• Transparent cover, such as a plastic lid, glass sheet, or cling wrap 
• Growing medium, such as grow plugs or seed starting soil 
• Bright lights, such as a grow light 
• A source of heat beneath seedling tray, like a seedling heat map 

What is the best seed starting container? The fastest, easiest way to get started is with a Gurney’s Seed Starting Kit to successfully start all kinds of seeds!

Read on for our recommendations for seed starting supplies.

Gurney’s Seed Starting Kit

Gurney’s Seed Starting Kit contains everything you need for fast germination and vital root growth: 

• 55 cells of soil-less Grow Plugs made of biodegradable materials, so they can be directly transplanted into the garden.  
• The lightweight Styrofoam Growing Tray, which makes it easy for seedlings to take up just the right amount of water. 
• The Humidity Dome, which holds in warmth and moisture to ensure early, uniform seed germination.  
• Seedling Boost Fertilizer to nurture healthy seedlings.  

When investing your time & care to start seedlings indoors, we also recommend using a grow light and a seedling heat mat. Without heat, cold temperatures could prevent your seeds from even germinating, and short daylight hours mean seedlings won’t stand a chance of healthy growth. A grow light and a heat mat are helpful tools to use year after year for seedling success! 

Seedling Heat Mats

Providing a heat source beneath your containers is a good way to increase germination. Tropical plants, like tomatoes, peppers, and eggplants,require warm soil for seeds to germinate. Bottom heat can be achieved by several means, but the easiest is the use of a heat mat like Gurney's Seedling Starter Heat Mat. Simply plug it in, place your seedling tray over the mat, and grow!

“Heating mats are a great investment. They're set to the perfect temperature to get the seeds sprouted quickly without cooking them. They provide just the right amount of gentle, continuous bottom heat. All my seeds are started on mats.” – Mike McGrath, garden author and You Bet Your Garden host

Grow Lights For Seedlings

Bright light is key to growing strong and healthy plants. Look for a growlight with an adjustable height, like Gurney’s Grow Light, so that you can keep the light as close as possible to your seedlings as they grow. Grow lights for seedlings allow you to mimic sunlight with 12-16 hours of full spectrum light each day, to help seedlings grow faster and develop stronger roots.

Building Your Own Seed Starting Kit?

Below are some recommendations for seed starting containers, trays,covers, and soilto complete your kit.

Seed Starting Containers

• What is the best seed starting container? Biodegradable pots, like these Peat Transplant Pots, allow you to transplant without disturbing the young plants growing inside because you plant the whole pot directly into the garden. These convenient vessels are made of peat, newspaper, or compost and will break down in the soil over time. A drainage tray will be needed for indoor watering. 
• You could also use convenient Peat Pellets, which give you a container and soil in one neat package. Peat pellets create an ideal substrate for seed germination. To get started, arrange the pellets in a tray, add water to expand them and sow your seeds. When the seedlings are ready to transplant, the whole pellet, outer bag and all, can be planted in the garden.  
• If using the Gurney’s Seed Starting Kit, it includes soilless plugs that can be planted directly into your garden. 

Tips: Seed starting containers can vary widely, so make sure they have drainage holes. Waterlogged soil in a poorly drained container will cause many problems, including rotted seeds and dead seedlings! Avoid using egg cartons for starting your seeds – they are not deep enough for healthy root growth.

Drip Trays

• You’ll need a shallow tray to contain and properly water your seed starting containers. Look for a shallow tray, such as a baking sheet, or use a purpose-built propagation tray or seed starting tray.

Transparent Cover

• Use clear plastic cling wrap, a sheet of clear plastic, acrylic, or glass, or a clear plastic cover to keep warmth and humidity contained for optimal seed germination.

Seed Starting Soil

• What is the best seed starting soil? If you’re starting seeds with Biodegradable pots or your own containers, start with a specific seed starting mix. Look for a natural or organic soil, especially when growing food. It's well worth the extra effort to start with a good quality seed starting mix, since a strong start carries over to better garden performance.  
• What to avoid: Do not use commercial potting soils with toxic chemical fertilizers. Do not use bagged soil that contains pine chips, wood fiber or other chunky material that can prevent seeds from breaking through the soil blend. Never use garden soil, because it is too heavy and can contain pests, diseases, and weed seeds. Avoid reusing potting soil since it can contain soilborne diseases. 
• If using the Gurney’s Seed Starting Kit or Peat Pellets, you will not need additional soil. 

 “Using a light, loose soil-free mix allows for the maximum root growth and good drainage that produces healthy plants; and beginning the season with healthy vigorous starts gives you your best shot at long-term success.”  – Mike McGrath, You Bet Your Garden host 

Fertilizer

• A dedicated seed starting fertilizer will include the ideal ratio of nutrients to support healthy seedling growth. With just ½ teaspoon of Seedling Boost Fertilizer per gallon of water, you can give your seedlings the nutrient boost they need. 

Step-by-Step Instructions for Starting Seeds Indoors

Gather your supplies and get started: let the germination begin! 

1. Prepare Your Containers 

• With your Gurney’s Seed Starting Kit, simply place the grow plugs into the cells, fill your water reservoir tray, and you’re ready to sow! 
• With a DIY kit: Moisten the soil and fill the pots. This can be done either by the placing soil into the containers and then watering them, or by moistening the soil in a bucket or tub and then filling the containers. Place your containers into your drip tray and fill the tray with water. 

2. Plant Your Seeds 

• With your Gurney’s Seed Starting Kit, simply sow 1-2 seeds into each pre-dibbled plug.  
• With a DIY kit: Make a small divot in the soil of each pot, the depth suggested on the seed packet. Carefully drop 1-2 seeds in each divot. After seeding trays or containers, cover the seed divot and seed with additional potting soil and gently water in using a misting bottle with a very fine shower. 
• When seeding plug trays or small cell packs, it's okay to have two or more seedlings germinate in the same cell, and thin to one plant later.  

3. Label Your Seeds 

• Using permanent marker, write on your container or use small plant tags to label what seeds you planted in each cell.  

4. Cover Your Seeds 

• With your Gurney’s Seed Starting Kit, simply place the humidity dome over the tray. This will hold in the warmth and moisture critical for germination. 
• With a DIY kit: place your cover over the containers   

5. Keep Seeds Warm 

• A seedling heat mat helps maintain steady warmth and speeds germination. Most vegetable seeds germinate well between 70-75°F. Warm-season crops such as peppers and eggplants benefit from bottom heat in the 75- 85°F range at the root zone. Cool-season crops including brassicas and lettuce prefer slightly cooler temperatures around 65-70°F.  

6. Keep The Light Bright 

• Position the tubes an inch above the plants. Arrange so that the lights can be raised or lowered as plants grow.  
• About 12-16 hours of light per day is ideal. Since that’s hard to get from your windowsill, we recommend using a grow light.  

7. Water Your Seedlings 

• Over the next week, you want to see water droplets beaded up on the inside of the cover every day, since humidity encourages germination. If you don't see water beading up, add water to your tray. Evenly moist but not sopping wet soil is your goal. Avoid watering from above, since that can disturb or dislodge young seedlings. Misting from above is beneficial too. 
• Note: In many cities, tap water may contain chemicals that can harm young seedlings. If possible, use rainwater, melted clean snow, or filtered water for the best results. 

Once You See Sprouts: 

• Remove Cover: After you see sprouts in at least half of your cells or containers, remove your humidity lid.  
• Warmth: Keep the heating mat on until all of your seeds have germinated, then remove it.  
• Light: Place the grow light as close as possible to the tray as you can. Be sure to keep your bright light on 14-16 hours/day. Your grow light can be plugged into an inexpensive timer to control light hours. Rotate your seedling trays under a light source so that the seedlings do not get spindly. 
• Watering: If the cells feel light, water them by adding water to your tray. Avoid watering from above. Do not let your tray remain filled with water for more than a couple of hours – empty excess if needed. Saturation for days at a time can encourage 'damping off' disease. 
• Fertilizing: After about 2 weeks, start using the Seedling Boost Fertilizer in your watering routine.

Watch: HowToUse Gurney’s Seed Starting Kit

From Thinning to Transplanting: What to Do After Your Indoor Seedlings Sprout

After germination, when seedlings start to grow, there are some important steps to ensure your tiny seedlings become healthy transplants. 

• You may need to thin multiple seedlings to make room for growth 
• You may need to pot up seedlings into larger pots 
• Hardening off is an essential step to reduce transplant shock 
• Then, it’s time to transplant them into your garden! 

Thinning Seedlings

• Thin seedings around 2 weeks after germination. Yes, it’s painful to trim your baby seedlings, but you must – otherwise your plants will suffer as they reach full size! 
• If only one seed sprouted in each cell, you're all set. But, if both seeds sprouted, you must thin them out to one plant per cell.  
• Keep the more robust sprout, with the best color and a short stocky look. Trim back the smaller seedling with a small pair of scissors—don’t pull them out, as that could disturb the other seedling’s roots.

Potting Up Seedlings

Plants that grow big and fast, like tomatoes and pumpkins, can benefit from 'potting up' into larger containers around week four or five. Plants that stay relatively small, like peppers, can usually stay in their original containers.  

(Reminder: Gurney’s soilless plugs and peat pellets can be directly transplanted into your garden.) 

• To up-pot most plants, place fresh potting soil in the bottom of the new larger container, place your plant plug in, and then fill in the sides up to the plug’s soil line.  
• When up-potting tomatoes (and tomatoes ONLY), place the bottom of the root ball at the bottom of the new container and then fill in the sides. (Always plant tomatoes deeply.) Continue to do this through any up-potting, up to and including planting.  
• Be very careful when handling the young plants. The roots, stems and leaves are very fragile at this stage. Plants sown and growing in individual plugs or cell packs may also need transplanting if they get root-bound.  
• It’s critical to follow seed packet directions about timing to avoid having to transplant – starting seeds too early may mean they out-grow their containers before you can plant them in the garden. See “when to start seedlings” below for more advice. 

Hardening Off Seedlings

Starting your seeds indoors allows them to grow in a controlled environment, where they enjoy a relatively stress-free life, protected from the heat, cold, wind, and rain. Hardening off is the process of acclimating young plants to outdoor growing conditions. This important step gives tender seedlings exposure to the elements gradually, so that they can be transplanted and live outside comfortably. 

• Start the hardening-off process 1-2 two weeks before the seed’s outside planting date, ideally when daytime temperatures are 50 degrees or warmer. 
• Set your seeds outside for a couple hours during the day in an area that’s shielded from sun and wind. Direct sun is a definite no-no for young seedlings at this stage because it will burn the tender leaves.  
• Bring your seedlings in at night.  
• Over the following days, increase the time your plants are outside – but do so gradually. Each day give them more and more sunlight, until they are used to being in direct sunlight all day.  
• After a few days of gradual and protected outside exposure, you can step up your efforts a notch. Leave your plants out longer and put them in sunnier, cooler, and windier spots.  
• Make sure the soil stays moist throughout the hardening off period – the seedlings may dry out faster than usual when outside exposed to sun and wind. But generally, you don’t need to feed or overwater your plants right now – remember, you’re trying to toughen them up! 

 Helpful Tips: 

• Peppers, tomatoes, melons, and eggplants are all tropical plants – they cannot tolerate cold nights. They will need plenty of time to acclimate.  
• Cole crops like broccoli and cabbage, and other cool-season crops, are more tolerant of cooler weather. 
• Tender crops need more hardening off, or acclimation to the elements, than cole crops and other cool-season crops.

Transplanting Seedlings Outside

When is time to transplant your seedlings? Don’t be fooled by looking at last average frost date' and daytime temperatures!  Instead, keep track of the nighttime low temperatures in the upcoming 10-day forecast. If you see any temperatures below 47 degrees, it’s still too cold to transplant. You will get the best yield from your garden plants if you wait to plant them outside until your 10-day forecast shows night time temperatures in the low 50s or warmer. Planting too early will set back your plants. 

• Water the ground thoroughly, and dig a hole just a few inches deeper than the pot in which your seedlings are currently residing.  
• Holding your seedling by the stem, place the seedling in the hole and cover it gently with soil.  
• Once the hole is filled, create a depression around the rim of the plant where water can collect. Be sure to water deeply. 
• If your weather hits an unexpected cold snap or heat wave, protect your seeds with sun shields, wind blocks, or Gurney’s cold covers. 
• Then wait a week to let them get settled before fertilizing or feeding. That food can be a 'mulch' of two inches of high-quality compost, or one inch of worm castings, or a balanced non-chemical fertilizer. If you're using a granulated fertilizer, spread the directed amount on the surface of the soil and then cover it with some compost or soil to activate it to release its nutrients faster. 

When to Start Seeds Indoors

When to start seeds is generally based on the date when your plants can be planted in the garden safely. Most plants should be started from seed approximately eight weeks before it will be safe to transplant them outside. Always read the instructions on your seed packets typically give instructions on timing, often described as “weeks before planting out.”

The timing for planting seeds can vary depending on the type of crops. For example, cole crops such as cabbage and broccoli can withstand a bit cooler weather, so they may be able to be transplanted before tender vegetables. Tender vegetables, such as plants like basil, peppers, and tomatoes need warm soil and warm evening temperatures to thrive, so they typically to go in the garden much later than cabbage, kale and broccoli.

When growing seeds indoors, it doesn't pay to start your seeds too early. This is true for several reasons: plants can get too big for their pots or their growing space before you can safely plant them in the ground.

Tip: If you’re new to gardening, consult your local extension service—many will offer detailed planting calendars for your local area.

“My rule is to start your seeds exactly two months before you intend to set the plants out. Research the historical weather data for your region for the past five April, May and Junes. That should give you an idea when NIGHTIME temps typically stay in the fifties.” -- Mike McGrath, garden author and You Bet Your Garden host 

The Best Seeds to Start Indoors

Many common vegetables such as tomatoes peppers, cabbage, broccoli, eggplant, and brussels sprouts, are well suited for starting early. Vegetables that originate from warm climates, like tomatoes, peppers and eggplant, need warm ground to grow and produce bountiful harvests, so the common practice is to start these seeds indoors and transplant the plants into the garden once the weather and soil are warm enough. Others, like cabbage, broccoli, and brussels sprouts, need a long time to grow for a bountiful harvest, so getting started early is helpful in climates with shorter growing seasons. 

Beans and peas are often recommended for direct sowing in the garden, but these could also be started indoors. Think of it as your seed safety net, in case the outdoors conditions prove unforgiving. Usually peas and beans are ready to plant out within a month or less - they have very vigorous root systems that fill the cells quickly. 

Some vegetables don't transplant well and are best directly sown into the ground. These include vine crops such as pumpkins, melon, and squash. Avoid starting root crops like turnips, beets, and carrots indoors.

Tips:  

• Tomatoes will grow faster and taller than your other starts, so plan for that! 
• If you want lots of peppers, start the seeds 90 days ahead of planting time instead of the traditional 60 and keep them in the brightest light  

Seed Starting FAQs

What are common seed starting mistakes?  

• Not enough light: Short daylight hours (less than 12 hours/day) mean seedlings won’t stand a chance of healthy growth. 
• Too Cold: Without heat, cold temperatures could prevent your seeds from even germinating,  
• Containers with no drainage: Waterlogged soil in a poorly drained container will cause many problems, including rotted seeds and dead seedlings 
• Poor quality soil: Commercial potting soils with toxic chemical fertilizers will harm seedlings. Bagged soil with pine chips, wood fiber, or other chunky material can prevent seedlings from breaking through. Heavy garden soil and can contain pests, diseases, and weed seeds. Re-used potting soil can contain soilborne diseases.  
• Too little moisture: If you don’t see water beading up on the inside of your cover every day, you’ll need more water to encourage germination. 
• Too much water: Saturated soil for days at a time can encourage 'damping off' disease. Don’t let your tray remain filled with water for more than a couple of hours – empty excess if needed.  

 Is it better to start seeds in trays or pots?  

Both are good options! For example, Gurney’s Seed Starting kit tray with soilless grow plugs is optimized for seedling germination, and the plugs can be planted directly in the garden without disturbing roots. Larger Peat Pots can also be transplanted directly into the garden without disturbing the seedling roots.