How to Grow Strong Seedlings: Expert Tips for Healthy, Thriving Plants

Growing strong seedlings is the start of a successful garden. Placing seeds in the soil is easy, but caring for them after they’ve germinated can be difficult.

The early stages of a plant’s life lay the foundation for its overall health and growth potential, making it extra important to provide the best possible conditions for seedlings to thrive.

The thrill of germinated seedlings is diminished when they all start dying.

Several factors contribute to the development of strong seedlings. These include the quality of seeds, the growing medium, and the environment in which they are growing.

Providing a well-balanced and nutrient-rich environment for young plants will encourage them to develop strong roots and sturdy stems, setting them up for success as they grow.

Also, proper lighting, temperature, and humidity all play a role in seedlings being healthy and robust from the start.

When I first started seedlings, I had a hard time growing them.

If I managed to keep them alive until I could put them outside, they usually pulled through. Before I implemented grow lights I killed a lot of plants!

Selecting Seeds

Importance of Seed Quality

Choosing high-quality seeds is very important for growing strong and healthy seedlings.

High-quality seeds are more likely to germinate, produce uniform plants, and have increased resistance to disease. 

When purchasing seeds, consider sourcing them from reputable suppliers who perform quality testing and adhere to good storage practices.

The right seed storage is important too, as it can directly impact the viability and vigor of the seedling. If you are planning to store seeds for several years, it’s a good practice to place them in a cool place. 

Varieties and Types

There are so many different plant varieties available today. Each offers different characteristics such as size, color, yield, and disease resistance.

When selecting seeds, it’s essential to choose plant varieties that are suitable for your local growing conditions and climate. This will increase your success rate.

Unfortunately, we in Alberta can’t grow all the stuff they do in Florida!

Consider factors such as the length of your growing season, average temperatures, humidity levels, and sunlight exposure when selecting seeds.

By choosing plant varieties adapted to your specific conditions, you increase the likelihood of growing strong and healthy seedlings.

Hybrids vs. Heirlooms

Seeds can be classified into two groups: hybrids and heirlooms. Understanding the differences between these categories can help you choose the best seeds for your garden.

  • Hybrids are plants created by crossbreeding two different parent plants to combine specific traits. Hybrid seeds often produce stronger, more vigorous plants with higher yields and increased disease resistance. However, the following plants will not produce seeds that are identical to the parent plants. This makes them unsuitable for seed-saving purposes. 
  • Heirlooms are open-pollinated plants with a long heritage, typically grown for generations within the same family or community. Heirloom seeds are known for their diverse flavors, unique appearances, and the ability to produce seeds that remain true to the parent plant. These seeds can be saved and replanted year after year, preserving their unique traits and characteristics. If you start saving seeds from your own garden, the following plants will be more suited to your community (which is kinda cool). 

When selecting seeds for your garden, consider the benefits of each type and evaluate which will be better suited for your specific growing conditions and preferences.

Preparing the Soil Mix

Ingredients and Ratios

The right ingredients in the right ratio will create a fertile seed starting mix. Ideally, it would compromise of:

  • 1 cup of potting soil
  • 1 cup of fine compost
  • 1 cup of worm castings
  • 1 cup of perlite

This combination provides a light, well-draining, and nutrient-rich environment for seeds to germinate and seedlings to grow. In a pinch, you can still plant seeds into a soil mix from a garden center. 

Adjusting pH Levels

Ensuring the appropriate pH level is critical for seedling growth, as it affects nutrient availability. Most plants prefer a slightly acidic soil mix with a pH of 6.0 to 6.8.

You can test your soil mix’s pH using a pH meter or a simple home test kit. If the pH needs adjustment, follow these guidelines:

  • To raise the pH, add small amounts of agricultural limestone (dolomitic lime)
  • To lower the pH, add small amounts of sulfur or sphagnum peat moss

Make adjustments gradually and retest the pH, as sudden changes in pH may harm the seedlings.

Planting Seeds

Planting seeds properly is essential for developing strong and healthy seedlings. Two factors to consider are sowing depth and spacing, and providing the right germination conditions.

Sowing Depth and Spacing

Seeds should be sown at an appropriate depth in the soil, which varies for different types of plants. A general rule of thumb is to plant seeds at a depth two to three times their width.

Read seed packets or refer to a reliable resource for specific planting depths based on the plant species.

  • Small seeds: Surface sow or cover lightly with soil (1/16 – 1/8 inch depth).
  • Medium seeds: Sow about 1/4 – 1/2 inch deep.
  • Large seeds: Sow 1 – 2 inches deep, depending on their size.

The right seed spacing is also crucial for avoiding overcrowding and allowing seedlings room to grow. Plant seeds at least one seed width apart from each other.

When seedlings emerge and develop their first set of true leaves, you can thin them by removing weaker plants to create appropriate spacing.

Proper Germination Conditions

For optimal germination, seeds generally require moisture, warmth, and in some cases, light. Keep the soil consistently moist but not soggy.

Use a spray bottle or mister to water the soil gently without disturbing the seeds. Cover the seeds with a plastic wrap or glass to maintain humidity if necessary.

Keep the cove on until the seedlings are about an inch tall. 

Maintain an ambient temperature of around 70°F (21°C) for most seeds. Place the seeds near a heat vent or use a heat mat to provide consistent warmth.

Note that specific germination temperatures may vary for different plants, so be sure to research the ideal temperature for the seeds you are sowing.

Some seeds need light to germinate, while others prefer darkness. Surface-sown seeds typically require exposure to light, whereas those sown deeper in the soil should be shielded from light. 

Providing Correct Light and Temperature

Light Requirements

Seedlings require enough light to grow strong and healthy. Lack of light is the number one problem I had with my seedlings.

That, and damping off. It is recommended to provide 12-16 hours of light per day for most seedlings, either using natural light or artificial grow lights.

Indirect sunlight is best as direct sunlight can be too intense and damage the plants. Fluorescent shop lights are recommended by some gardeners, as they offer a consistent light source.

If using grow lights, it’s important to adjust the height of the lights as the seedlings grow, ensuring they are not too close or too far away.

Keeping the lights too close can result in scorched leaves, while keeping them too far can lead to weak, leggy growth.

Temperature Ranges

Temperature also plays an essential role in seedling growth, as it determines the germination rate as well as the overall health of the seedlings. Most seeds germinate within a temperature range of 60°F to 75°F (15°C to 24°C). 

Temperature RangeSeedlings
60°F to 65°F (15°C to 18°C)Some cool-season vegetables, such as lettuce and spinach
65°F to 75°F (18°C to 24°C)Most common vegetables and flowers
75°F to 85°F (24°C to 30°C)Warm-season vegetables, like tomatoes and peppers

A simple way to achieve this is by using a heating mat or thermostat under the seed trays to ensure a consistent temperature.

It is crucial to monitor the temperatures closely and adjust the settings as needed to keep the seedlings healthy.

Watering and Fertilizing

Watering Techniques

Proper watering will also help you grow healthy seedlings. Initially, use a spray bottle to mist the soil as it produces a fine spray that won’t disturb or displace small seeds.

As the seedlings grow, you can switch to using a regular watering can. Check on your plants at least once a day and water any time the top layer of the soil starts to look dry.

If you have your seedlings in cups, you can easily feel the amount of moisture in the cup by lifting it. If the cup feels light, they need water. If they are heavy, they have enough moisture.

Monitor the moisture levels carefully to avoid overwatering or underwatering, both of which can harm the seedlings.

Nutrient Needs

Seedlings require appropriate nutrients for healthy growth. Choose a fertilizer that is high in phosphorus, as this nutrient promotes strong root growth and aids in photosynthesis.

Immature plants require more phosphorus than mature plants for optimal health. Examples of suitable fertilizers include water-soluble fertilizers and fish emulsions. 

Follow the directions on the chosen fertilizer to determine the correct amount and dilution with water.

However, you can also use alternative organic materials, such as compost or worm castings, to supply essential nutrients in a more sustainable manner.


Fertilizing seedlings at the right time is crucial for their healthy growth. Start by observing your seedlings and identifying the ones with at least four true leaves.

True leaves are the second set of leaves to emerge and different from seed leaves (cotyledons), which are not considered true leaves.

When you have identified the seedlings with four or more leaves, you can begin applying the diluted fertilizer. Water from the bottom if possible to promote proper nutrient absorption and prevent over-saturation.

Continue to fertilize and water regularly, following the recommendations on the product label, to support the ongoing development of your seedlings.

Fertilizing my seedlings is an area I still struggle with. I add cold brewed black coffee to my watering pot to give them a boost of nitrogen. 

Monitoring Growth and Troubleshooting

As seedlings grow, it’s important to keep a close eye on their progress to ensure they develop into strong and healthy plants.

Check on your plants once a day, monitoring their moisture levels, and their growth. 

Identifying Problems

There are several common issues that can arise with seedlings, and addressing them as soon as possible will ensure the best possible growth.  Here are a few signs to look out for:

  • Leggy growth: Seedlings that appear tall and thin may not be receiving enough light. Ensure they are under grow lights for 14-16 hours per day or outside in direct sunlight for 6-8 hours daily.
  • Slow growth or pale yellow leaves: This could be a symptom of inadequate nutrition. Begin feeding your seedlings with a weak dose of liquid fertilizer and gradually increase the strength as they grow larger.
  • Wilting: If seedlings are wilting, they may be overwatered or underwatered. Check the moisture level in the soil and adjust your watering schedule accordingly. When we left home for 5 days, a lot of my plants were wilted. A good watering rescued most of them. 
  • Poor germination: If the germination rate for your seeds is below 70%, consider purchasing higher-quality seeds or adjusting your germination environment. 

Common Pests and Diseases

Seedlings can be susceptible to various pests and diseases as they grow. Here are some common issues to look out for:

  • Damping-off: This fungal disease often presents as seedlings rotting at the base or collapsing. To prevent damping-off, ensure your growing medium has good drainage, avoid overwatering, and maintain proper airflow around your seedlings. I always keep a fan running near my seedlings. This helps a lot to prevent fungal diseases. 

  • Aphids: These small insects can cause distorted or yellow leaves on your seedlings. To manage aphids, use insecticidal soap or introduce beneficial insects like ladybugs into your garden.

  • Spider mites: Tiny, spider-like pests that can cause leaf damage and speckling, spider mites can become a major issue if left unchecked. Manage them by using a miticide or releasing predatory mites to tackle the infestation.

  • Fungal diseases: Seedlings can be affected by various fungal diseases, such as powdery mildew or leaf spots. To prevent and manage fungal issues, ensure proper air circulation, avoid overwatering, and use organic fungicides as needed.

Transplanting Seedlings

Transplanting seedlings is a crucial step in the gardening process. To do it successfully, gardeners must ensure that their young plants are strong, healthy, and prepared for the transition. This section will cover the hardening off process and transplanting techniques to help your seedlings thrive.

Hardening Off Process

Before transplanting seedlings, they need to be gradually acclimated to the outdoor conditions through a process called “hardening off.” This prevents shock and damage to the delicate plants. Here’s a step-by-step guide to hardening off your seedlings:

  1. Start by placing your seedlings outside in a shaded, sheltered area for a few hours a day. I start with 1 or 2 hours max.
  2. Gradually increase the time spent outdoors each day over the course of a week or two. Take time for this.
  3. Begin to expose the seedlings to more direct sunlight and wind gradually.
  4. Once the seedlings can remain outdoors for an entire day and night without showing signs of distress, they are ready to be transplanted. 

Transplanting Techniques

Transplanting your seedlings to the garden should be done carefully to avoid damaging their fragile root systems. Follow these transplanting techniques to ensure successful growth:

  • Choose a cool, cloudy day for transplanting to minimize stress on the young plants. Planting in the late evening helps them to acclimate before the next days hot weather. 

  • Prepare the soil in the transplanting area by loosening it and ensuring it’s free of weeds and large clumps.

  • Water the seedlings in their pots before transplanting so the soil is moist but not soggy.

  • Gently remove the seedling from its container, taking care not to damage the roots or stem.

  • Place the seedling in a hole, maintaining the same soil depth as when it was in the pot, unless you’re planting them deeply for a purpose (as in tomatoes). 

  • Fill in the hole with soil, gently tamping it down to ensure good contact between the roots and soil.

  • Water the transplanted seedling thoroughly to help it establish itself in its new environment.

By following the hardening-off process and using proper transplanting techniques, your seedlings are more likely to survive and thrive in their new environment.

Take your time with hardening off. Trying to speed up this process will cause seedlings to suffer. 

Growing strong seedlings can be challenging as a new gardener, but don’t give up. Start by buying seeds from a reputable source, buy or make a good seed starting mix, and follow watering techniques and light requirements.

You’re well on your way to growing strong seedlings and will soon be enjoying healthy thriving plants.

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