Growing seedlings (baby plants) indoors helps to produce stronger, healthier plants.
Our gardening team covers everything you’ll need to know about growing seedlings indoors.
You will learn how to set up, grow and transplant your seedlings. We’ll start by looking at the benefits of growing seedlings indoors and the best plants to start growing indoors. After that, we’ll cover all the practical steps.
Finally, we’ll explain how to prepare your seedlings for the outdoors and how to transplant them.
So, let’s get started…
The Benefits of Growing Seedlings Indoors
The benefits of starting to grow your seedlings indoors include the following:
- Maximize the growing season. If you live in a temperate climate, you can grow your seeds indoors during winter. By the time winter is over, your plants would have gotten a head start. They will be ready for you to transplant them outdoors, as soon as spring begins.
- Earlier harvesting. If you plant at the very start of the growing season, you will be able to reap sooner.
- Better germination. By starting indoors, you will have better control over the environment. This allows you to create the most suitable conditions for the seeds to germinate.
- Healthier plants. Once seeds have germinated, you can adjust the indoor conditions to make it favorable for the seedlings to develop and flourish.
- Minimize cost. It is much cheaper to buy seeds and growing materials than to purchase plants from a garden shop.
- Boost Your Gardening Expertise. By growing your seedlings, you will gain practical knowledge and experience. So, your plants and your gardening expertise will both flourish.
Best Plants To Start Growing Indoors
Generally, plants with slow root development and those that cannot tolerate cold temperatures can be started indoors. Examples of plants that can be started indoors include:
- Brussels Sprouts
Plants that grow very quickly and can tolerate some cold, can be planted directly outdoors. Also, plants that do not grow well after being transplanted, can be planted directly outdoors. Here are some examples of plants that can be planted directly outdoors:
The Best Time To Start Seeding Indoors
If you live in a tropical climate (where there is no winter), you can start seeding 1-2 months before the dry season begins.
If you live in a temperate climate, timing is crucial. If you start growing your seeds indoors too early, they’ll become too big before they can be transplanted outdoors.
But, if you start too late, they will be too small for transplanting, when the growing season begins. Consequently, you won’t be able to fully utilize the growing season.
Ideally, you should aim to begin indoor seeding 6-8 weeks before the last frost date in your area.
Not sure when the last frost date is in your area? Check The Old Farmer’s Almanac. 
Get The Right Containers
If you’re on a budget, you don’t have to spend money on seedling containers. You can use empty plastic containers, such as empty yogurt cups, ice cream containers, or food containers. But make sure you clean them properly. Also, they should be at least 3 inches deep to accommodate the plants’ roots.
Alternatively, you can buy seed-starting containers from your local garden shop or online. Seedling containers, such as seedling flats, plug trays, or peat pots, are designed especially for this purpose. So they are easy to use.
Peat pots are a bit more expensive but they are biodegradable. This means that you won’t have to remove the seedlings from the container to transplant them. When the time comes to move your plants outdoors, just place the containers (with the plants in them) into the ground.
The disadvantage is that you won’t be able to reuse the containers.
Prepare The Containers
If you are recycling containers, it is important to have them properly cleaned and prepared for the new seeds.
Contaminated containers can be a source of disease, which can be detrimental to young plants.
If you are using empty food containers, ice cream containers, etc. you’ll also need to puncture some drain holes at the bottom of the containers.
If you are seeding different crops at the same time, you should label the containers. This will help you to easily identify the different plants since baby plants tend to look alike.
Get The Right Seeds
Seeds should be strong and resilient. This helps to maximize the chances of producing healthy seedlings.
Seeds should be well developed and mature. They should also be free from pests and diseases.
The best way to help ensure that you are getting healthy, good-quality seeds is to buy them from a reputable source. They usually have good quality assurance measures to ensure that seeds are mature, properly treated, and disease-free.
Also, when you open the seeds packet, you should examine the seeds for any obvious signs of damage, disease, or deformity.
Get The Right Soil
The best soil to use for growing seedlings is a seed-starting mix.
Seed-starting mix is sometimes referred to as seed-starting soil. The fact is, it does not contain any soil. It is a compound of materials that usually contains peat moss, vermiculite, coconut coir, composts, and perlite.
Seed-starting mix has a light texture that makes it easier for young plants to burst through the surface, after germination. It is also less likely to contain pathogens and contaminants that are usually found in actual soil.
You can use a potting mix to start growing your seeds but it is not ideal. It is denser than the seed-starting mix and makes it more difficult for the seeds to germinate and break through the surface. It also has a greater risk of containing soil-borne diseases.
Prepare The Soil
If you use a seed-starting mix or potting mix, you won’t need to do a lot of preparation.
But before you pour the soil/ soil-less compound into your seedling containers, pour it into a bucket or large container. Then add some water to it and mix it up well, so that it becomes moist and has an even texture. Try not to make it too soggy, though.
By doing this, you will remove the air pockets from the soil/ soilless mix. It also makes the soil nice and compact, so it won’t shrink when it is placed into the seedling containers.
How Deeply Should Seeds Be Planted?
The depth at which seeds should be planted varies. The seed packets usually indicate how deeply to plant the seeds.
As a general guide, the larger the seed, the deeper it should be planted. Most seeds can be planted at a depth of less than 1 inch.
Some seeds only need a light covering of soil/ seed-starting mix to keep them moist and minimize exposure to light, during germination.
Some seeds, like basil, can even be left uncovered.
Always refer to the information on the seed packets for guidance on the appropriate depth at which the seeds should be planted.
Cover Containers During Germination
Covering the containers during germination helps to preserve the moisture and temperature.
If you are purchasing your seedling containers, look for those that are already outfitted with a plastic cover or dome.
If your containers don’t have a cover, you can use ordinary plastic wrap to cover them. Try not to let the plastic rest on the surface of the soil/ seed-starting mix. Leave some space for the plants to break through the surface, after they germinate.
Once germination is completed and the plants begin to burst through the surface, remove the covering.
Manage The Lighting
During the germination period (1-2 weeks) most seeds require very little light. So at this stage, keep them away from direct sunlight or other bright lights.
However, once seeds have germinated and they start growing, lighting becomes much more important.
Seedlings require significant amounts of light to grow strong and healthy. At the minimum, they should be getting about 6-8 hours of light, at the very early stages of growth.
After they begin to produce true leaves, lighting can be increased to 12-16 hours per day.
Side Note: The first leaves (or leaf-like structures) that appear soon after germination are not true leaves. They are known as cotyledons. Cotyledons store food for the plants before the true leaves appear. True leaves are the leaves that start appearing after the cotyledons.
To ensure that maturing seedlings get enough light, you may need to use artificial lighting, such as grow lights. If you use grow lights, place them a few inches above the seedlings. As they begin to grow you’ll need to adjust the lights accordingly, so they won’t be too close to the plants.
Plants lean towards the light. If your plants are on a windowsill or close to a window, you’ll need to rotate them often, so they won’t lean and stretch in one direction towards the light.
Manage The Temperature
Plants generally need more heat during the germination period than they do after germination.
Before the seeds germinate, try to maintain a temperature between 75-80 degrees Fahrenheit (24-27 degrees Celsius). If the indoor space is not warm enough, you can use a heat mat to create and maintain the right temperature for the seed to germinate properly.
Once the seeds germinate and begin to grow, you can reduce the temperature to about 65-70 degrees Fahrenheit (18-21 degrees Celsius).
When To Start Watering
For most plants, you shouldn’t need to water the seeds during the germination period. If you properly water the soil before planting the seeds and cover the seedling containers, the cover should maintain the moisture.
Once the seeds have germinated, remove the cover from the containers.
Watering during germination can sometimes cause the seeds to shift around, and this could affect germination. But you should still check the soil often to ensure that it remains moist. If it is dry to the touch, water it… preferably with a mister or spray bottle.
After germination and the seedlings begin to grow, they need to be watered regularly (especially as they begin to mature).
Watering seedlings once or twice per day should be sufficient. The idea is to keep the soil consistently moist… not too wet and not too dry.
It’s best to water them in the mornings and check them in the evening. If the soil is still moist in the evening you won’t need to water them until the next day.
Keep in mind, as seedlings begin to mature they will require more water. For more information on watering plants correctly, check out our article: “10 Golden Rules For Watering Plants“
When To Add Fertilizers
During the germination period, no fertilizer is needed.
After germination and the baby plants begin to grow and produce true leaves, they’ll need some fertilizer. This is especially so if you use a soilless mix, which has no nutrients.
Generally, you can begin to apply fertilizer 1-2 weeks after germination and the seedlings are 1-2 inches tall.
Use a liquid fertilizer and apply it only once per week, or as often as the fertilizer’s instructions indicate. Homemade fertilizer can also effectively provide plants with the nutrients they need.
Protect Seedlings From Diseases
If the right conditions are not provided, seedlings can fall victim to diseases.
To minimize the risk of diseases, consider the following measures:
- Clean containers – If you are using empty food containers or if you are reusing seedling containers, ensure they are properly cleaned before placing soil or soilless mix in them. This helps to get rid of contaminants that can cause plant diseases.
- Use soilless seed-starting mix – Although you can use a potting mix to grow your seeds, it increases the risk of soil-borne diseases. The risk is much lower with soilless seed-starting mix and it is less dense than potting mix. This makes it ideal for germination and growth.
- Water adequately – Too much water can inhibit oxygen and lead to diseases. Likewise, too little water deprives the plants of moisture and can cause disease and death. Keep the soil evenly moist, not too wet, and not too dry.
- Ensure there is good air circulation – This helps to keep the stems robust and flexible and minimizes the risk of diseases. You can use an ordinary fan, but don’t aim it directly at the plants. You can aim it just above the plants. Also, the breeze should be gentle, so keep the fan at a low speed and not too close to the plants.
If you are planting large seeds, you can plant 1 or 2 seeds in each space of your seedling container.
For smaller seeds, you can plant multiple seeds in a single space. This maximizes the opportunity for at least some of them to germinate. If you plant just one seed and it doesn’t germinate, you have effectively wasted that space and other resources.
If you plant multiple seeds in the same space in your containers and most or all of them germinate, you may need to remove some of them after they sprout. Why? We’ll explain below.
Thinning is the term used to describe the process of removing the weaker seedlings from a cluster of seedlings. This allows the stronger ones to have a better chance of healthy development and growth.
You can thin seedlings after they begin to produce true leaves and it is easy to distinguish between the weaker and stronger ones.
To thin seedlings, you can remove the weaker plants by pulling them out of the soil. But you’ll need to be very careful not to disrupt the soil of the remaining plants. The easier method for thinning is to simply cut the stem of the weaker plants with a scissor or small garden shear.
Okay, so you’ve gone through the entire process and your seedlings are now growing nicely. What’s next?
The next step is to get them ready for the outdoors. This preparation is known as hardening off.
The outdoor conditions are much harsher than indoors. If you simply took your seedlings from indoors and transplanted them outside, some of them may not survive the sudden change of environment. They need time to get use to the outdoor conditions before they are moved there permanently.
Here’s how hardening off works…
About 1 to 2 weeks before seedlings are ready to be transplanted outdoors (i.e. 1 to 2 weeks before the final frost date), move them outdoors for a few hours each day.
Initially, don’t expose them to direct sunlight, wind, rain, etc. If you have a partly shaded area outside that is not exposed to strong wind, that might be the best place to initially put them.
For the first day or two, keep them outside for about 1 to 2 hours then bring them back indoors.
After that, you can gradually increase their outdoor exposure by leaving them an extra hour each successive day. By day 3 – 4 you can also begin to expose them to some direct sunlight.
If you continue this process, they should be adequately prepared to cope with the outdoor conditions, permanently.
They should now be ready for the final stage of their journey… transplanting.
Transplant To Outdoor Environment
Generally, most plants can be transplanted approximately 6-8 weeks after the seeds were first planted indoors.
Transplanting is the final stage of the journey from seed to plant. At this stage, you should have healthy seedlings that have gone through the hardening-off process.
During the hardening off period or just before, begin to prepare the area outside where you intend to grow your plants. Make sure it is cleared, tilled, has the right soil composition, and is sufficiently moist. If you are using raised beds, make sure they are set up and prepared for the seedlings.
The best time to transplant seedlings to the outdoor environment is early in the morning.
Transplanting is relatively straightforward.
For each seedling, dig a hole deep enough to accommodate its roots and soil/ soilless mix. Carefully remove each seedling from the container, keeping the soil or soilless mix and roots intact. Place them in the holes and cover the roots properly. Ensure they are firm enough in the ground, so they won’t topple over with the wind.
Remember to sufficiently space your seedlings, so they’ll have enough room to expand as they grow.
If you’ve gotten this far, congratulations! Well done!
Now that you know how to grow and transplant seedlings, you need to care for them over the next phase of their development and growth.
If you’re new to gardening, check out our article, “how to start a garden“, for some great tips.