How to Start Strong Plants from Seed Indoors

Most of us begin our gardening season by planting some of our vegetable seeds indoors. However, many of us find it to be a very frustrating experience.

When I was first learning how to garden, I can’t tell you how many times my poor seedlings failed to grow at all, or became extremely tall and leggy and weak. Sometimes they died suddenly in their containers, while others died shortly after being planted outside.

It took me many years of trial and error to learn how to successfully start my vegetable seeds and grow healthy transplants. Here are the top tips I’ve learned to start strong plants from seed indoors:

Use Vigorous Seeds

First of all, it’s important to use strong and vigorous seed. Old or poor quality seeds often fail to germinate well, take longer to grow, and may produce weak seedlings.

Start Them at the Right Time

Don’t start your seeds too soon! Holding your plants for too long in containers while waiting for the weather to warm up can cause your plants to become root bound. They may never recover and grow well when finally planted in the garden. It’s often better for you to plant them a couple of weeks later than normal in the spring, than for you to start them too early. Johnny’s Seed Starting Date Calculator is a great tool to help determine your best starting dates.

Use Good Quality Soil Mix

Use good quality seed starting soil mix. There are excellent mixes available from Johnny’s Selected Seeds, Gardener’s Supply Company,  Peaceful Valley, 7 Springs Farm, and other companies.

You can also use many recipes for creating your own soil mix.  Here’s one simple formula:

3 parts peat moss or coir
1 part perlite or vermiculite
2 parts compost
For every 15 gallons (56 liters) of soil mix, add 2 cups (.47 liter) organic fertilizer blend

Most potting soils are very dry when you purchase them.  You need to thoroughly moisten the soil BEFORE you put it into containers for planting. It takes a lot more water, effort and time to properly re-moisten the soil than you might realize, as dry peat moss strongly repels water. Put the dry potting mix into a large bucket, dishpan, tray or other container that will give you plenty of room to mix and turn the soil over as you add water to it. It should feel like a moist, rung-out sponge when it’s ready.

Your local garden centers often carry a sterile soil mix made of peat moss or coir, with vermiculite or perlite – but no compost or fertilizer. This sometimes has a small amount of lime added, if peat moss is the main ingredient. But these mixes often don’t include any fertilizers, so you’ll need to either add a fertilizer blend to it, or start using a liquid fertilizer (half strength) once a week to provide nutrients to your young plants as soon as they start forming true leaves. However, most organic liquid fertilizers are fish-based, and I don’t particularly like the smell of that inside my home.

Make sure that your compost is fully mature and not contaminated with herbicides (read Don’t Kill Your Garden with Compost). Immature (unfinished) compost may contain compounds that will damage your seedlings or prevent seeds from germinating well.  If you use vermicompost (earthworm castings), limit it to 10% of the soil mix (9 parts mix with 1 part vermicompost).  Higher concentrations can stunt the growth of young plants.

Grow Your Transplants in Larger Containers

You can choose among many different types of containers to start your plants.  I suggest that you grow your seedlings in larger pots than nurseries usually use. (Read One Way to Increase Yields up to 50%)  I prefer to use 3” wide by 4″ tall (7.5 cm) containers for most vegetables, but 4” wide (10 cm) containers for tomatoes, squash, and other large vegetables. But if you don’t have room to use larger pots, you can still successfully grow seedlings in smaller ones – just plant them a little earlier, before they become root bound.

You can also use 8 ounce (.24 liter) paper cups, if you punch drainage holes in the bottom.  I don’t generally recommend using peat pellets or peat pots, nor growing plants in tiny “cute” eggshells.

Fill containers to about ¼” (.6 cm) below the rim of the container, and gently firm the soil.  If the surface of the soil is too far below the rim, the young seedlings will have poor air circulation and may be more susceptible to fungal disease. To reduce problems with fungal disease (damping off), some people cover the seeds with vermiculite or shredded sphagnum moss (not peat moss) instead of the potting soil.

Give Your Seedlings Elbow Room

If seeds are too crowded, they will become leggy very quickly in their effort to outgrow their neighbors as quickly as possible. I usually plant 2-3 small seeds (or 1-2 large seeds) in each pot, and I space each seed in the pot at least ½-1” (1-2.5 cm) apart. As soon as the plants start touching each other as they grow, I keep the most vigorous healthy seedling, and cut the others down to soil level (don’t pull them out, which can damage the roots of the remaining plant).

Provide the Right Amount of Warmth

Provide enough warmth for your seeds to germinate quickly. Cool-season crops like lettuce and cabbage usually germinate fastest at around 70 F (21 C), while warm-season crops like tomatoes and peppers prefer closer to 80 F (27 C).  Most seeds will still germinate at lower temperatures, but they may take much longer to do so (such as 10 days instead of 4 days) and they may be more susceptible to fungal disease during that period.

Use heat mats under the seeds, if necessary, to provide extra warmth in cool rooms.  In this case, you can start the seeds in small flats on the heat mats, and then transplant the young seedlings into larger containers once they start growing their true leaves.

If I’m not using heat mats, and I’m using a light stand with two levels of lights, I’ll put my newly-planted seeds under the top lights, where they’ll get some extra heat rising from the lights below.

Most seedlings will grow better at slightly lower temps than what they prefer to germinate at, so don’t keep them too hot after they’ve sprouted.  Keep the soil moist, but not soggy wet.

Provide 14-16 Hours of Bright Light

I don’t usually recommend growing seedlings in windows. While some people are successful at it, windows have cold drafts and often don’t provide bright enough light for enough hours of the day to produce strong seedlings.

Instead, I recommend growing your plants under long T5 or T8 florescent bulbs, which are brighter and more efficient than the old T12 bulbs. I’ve had problems with the cheapest brand of shop lights used to hold the bulbs, and now prefer to purchase the medium-quality shop lights, which are usually about $20.

Any commonly-available type of florescent light bulbs (warm or cool, but not compact) can grow good seedlings, but bulbs more than 2 years old might not provide enough light.  Keep your seedlings only 2-3” (2.5-7.5 cm) directly underneath the lights, and provide 14-16 hours of light every day – but not 24 hours! Seedlings need darkness during the night. Using a timer makes this easy.

There are a number of newer LED lights in long tubes.  Some of them may work well for starting seeds, but please do some research on them first.  I don’t have any experience in using them.

Light stands are often expensive, but it can be cheap and easy to create your own. Here’s directions, with a video, on how to create a light stand out of inexpensive PVC that can be taken apart and put away when not needed.

Harden Your Plants Before Putting Them into Your Garden

Once your seedlings reach 2” (5 cm) high, lightly brush the young plants with your hand (10 times, once a day), to encourage stocky growth.

Plants grown indoors are not used to sudden temperature changes, wind, or direct sunlight. If you don’t “harden them off” before putting them into the garden, they can become sunburned or experience shock from the drastic change – sometimes badly enough to kill the plants. Start hardening off your plants about one week before the seedlings will be ready to be transplanted into your garden.

This process involves gradually exposing your plants to more sunlight, wind, and temperature changes. The simplest way I’ve found to do it is to protect the seedlings with heavy row cover on hoops for the first day or two. Then I open the row cover on the east side during the day just enough to allow the plants to receive 2-3 hours of direct early morning sunlight, but still be protected from most wind.  I fully cover them again during the night.

After a couple more days, I open the row cover enough to expose them for ½ day. After a couple more days, I open it even more. After one week of this process, they have adjusted pretty well to being outdoors, and are ready to be planted into the garden. Sometimes I follow this process with new transplants put directly into the garden bed, instead of hardening them off in their pots first.

Plant Them at the Right Time

Don’t wait too long to transplant your young plants into your garden! Even delaying just 7-10 days past the ideal time can be enough to cause the plants to become root bound and stunted. Most plants are ready to be planted when they have at least four adult leaves, and their roots fill the pot just enough to hold the soil together – but before many roots start wrapping around the inside of the containers.

 

Summary

By following these simple tips, you can grow strong and vigorous transplants for your garden. Vigorous vegetable seedlings will grow quickly, produce more food, and may be more resistant to pests and diseases.