Growing Strong Seedlings

By Harley Smith
Published: March 1, 2016 | Last updated: April 26, 2021 11:46:02
Key Takeaways

A healthy, productive indoor garden cycle starts with strong seedlings. Here are a few basic propagation tips to help your plants reach their true genetic potential.

Caption: Close up of Zucchini Squash Seedlings Growing Source: AnnaHoychuk/Shutterstock

Giving your plants a strong head start is an important first step towards achieving success in your indoor garden. If your seedlings are weak and spindly, they may not make it off the propagation bench. Even if seedlings do survive, they may never reach their true genetic potential, so following a few propagation basics is well worth the time and effort.


Soaking seeds in water before plantingSoaking seeds in water before planting. Source: Gheorghe Mindru/Shutterstock

Soak Yours Seeds Before You Start

Whenever possible, it’s best to use fresh seeds that have been stored in a cool, dark place. Fresh seeds are loaded with starches and stored nutrients, and typically have a good germination rate when given adequate moisture and warmth. If you have older seeds or seeds that haven’t been stored under ideal conditions, a few extra precautions should be taken.


Kelp extracts make a great seed soak for older seeds, as kelp is loaded with natural growth hormones and contains many beneficial trace elements. One of the hormones found in seaweed extracts is called gibberillic acid. In nature, gibberillic acid triggers the release of enzymes in the seeds that begin the germination process. Soaking seeds overnight in seaweed extracts may improve germination rates by up to 30%.

Read also: Soup Up Your Soil With Kelp, Molasses & Guano

place your seeds into rockwool for a better watering and growingPlace your seeds into rockwool for a better watering and growing. Source: masterpict/Shutterstock


In hydroponic applications, stonewool starter cubes are a good choice for starting seeds. A sheet of stonewool starter cubes fits perfectly in a standard nursery tray, and it has the perfect air- to water-holding capacity. Just make sure you condition the stonewool with pH-adjusted water before planting.

For best results, soak the starter cubes for a few minutes in water with a pH of about 5.5. The conditioning solution will neutralize the limestone dust left over from the manufacturing process and provide plenty of moisture for germinating seeds. Pour off the excess water so the stonewool isn’t sitting in a puddle of water, then simply plant one seed in each starter cube.


Read also: pH Balance for Efficient Nutrient Uptake

LED Grow Light Full Spectrum lamps over seedsLED Grow Light - Full Spectrum lamp over seeds. Source: Bobkov Evgeniy/Shutterstock

Give Your Seedlings Some Full-spectrum Light

It’s also a good idea to cover the propagation tray with a humidity dome and place the tray under full-spectrum lights. The dome will keep the relative humidity in the propagation tray at about 98%—ideal for germinating seeds—and the full-spectrum lights will help keep the seedlings from stretching once they germinate. Seedlings don’t need much light intensity. Two standard fluorescent lamps over a nursery tray are adequate, but many gardeners, including myself, prefer four lamps.

Full-spectrum fluorescent grow lamps work best. The blue end of the spectrum helps seedlings remain short and stocky, and the red end of the spectrum helps stimulate root growth. Just keep the lamps a few inches above the growing tips and raise the lamps as needed. Also, remember to remove the humidity dome after all of the seeds have germinated and started to grow. Keeping the humidity dome on too long can set up an environment for fungal pathogens.

Read also: How a Fungal Outbreak Can Destroy an Entire Plant Cycle

Give Your Seedlings Some Bottom Heat

Bottom heat is also beneficial, especially for germinating tropical seeds. Most heating mats are set at the factory to provide a temperature of 78°F. The warm temperature helps activate enzymes such as amylase, the enzyme that breaks down starches into sugars, which provide energy for cell division and growth.

Remember not to use bottom heat when germinating cool-weather crops such as lettuce and spinach. Warm temperatures actually inhibit germination in cool-weather crops. Check your seed packages for any other special considerations. For example, some seeds may need to be cold-treated before they can germinate, and a few native seeds need to be scratched or treated with acids before they can germinate. Fortunately, most garden variety seeds are easy to germinate with little special treatment.

After the seeds germinate, the bottom heat should be reduced or removed. The warmer the water temperature, the less dissolved oxygen the water can hold. Remember, it’s not overwatering that kills the plant, it’s lack of oxygen. If the water temperature exceeds 78°F, it can’t hold enough dissolved oxygen and it sets up an environment for anaerobic fungi. Anaerobic fungi grow in stagnant, oxygen-deprived water, and they are the bad guys that cause root rot. After the seeds germinate, try to maintain the water temperature between 68 and 75°F for best results.

Read also: Germinating Seeds and Caring for Seedlings

Seedling tray where first true leaves have developedSeedling tray where first true leaves have developed. Source: New Africa/Shutterstock

Starter Fertilizers for Seedlings

Seeds contain all of the minerals and nutrients necessary to begin growing, so they can be germinated with water only. Once the first true leaves start to appear, however, they need a starter fertilizer. The first leaves that pop out of the seed are not true leaves; they are the embryonic leaves that are formed inside the seed. The second sets of leaves are the plant’s first true leaves. When they appear, it means the seedling is actively growing and that it is ready for a mild fertilizer.

During the first few weeks, use a full-spectrum fertilizer that includes trace elements such as iron, copper, manganese and zinc. The trace metals are cofactors that actually turn on enzymes used for photosynthesis and cellular metabolism. Also, make sure the pH is kept in the slightly acidic range, somewhere between 5.8 and 6.4. If the pH rises much past 6.5, iron starts to become unavailable to the plant. Iron is a catalyst for the formation of chlorophyll, essential for photosynthesis.

Read also: Building an Effective Fertilizer Regimen

If the pH rises above 7.5, all of the metal catalysts start to become unavailable and can slow down new growth. Zinc, for example, activates the enzymes necessary for indole-3-acetic acid (IAA) production—the growth hormone responsible for initiating root production.

Adequate phosphorus is also important for faster root strike and better establishment of the plant. Phosphorus is the energy element. It energizes the rooting process, and extra phosphorus is included in most commercial starter fertilizers. With just a little extra phosphorus, it is possible to see as much as 20% more roots.

Tomato Seedling in the hand with visible rootTomato Seedling with visible root. Source: Lyona/Shutterstock

Root Stimulants for Seedlings

Starter fertilizers provide all of the essential minerals that seedlings need, but organic bio-stimulants aid with the uptake of minerals. For example, humic and fulvic acids aid with the uptake of iron and other trace minerals. Humic acid is an intermediate chelator. “Chela” means “claw,” so humic acid molecules attach to mineral ions like a claw, holding them tightly enough to keep them from getting locked up in the soil, but loosely enough to release them to the root hairs on demand. Humic and fulvic acids also help to neutralize pH. In nature, humic acids raise the pH of acid soils and lower the pH of alkaline soils.

For even better results, combine humic acid with seaweed extracts. When used together in a 5:2 ratio, humic acid and seaweed work 50% better than either product alone. The combination stimulates root development, producing more lateral root growth and more root mass. Seaweed extracts also contain B-vitamins and amino acids that stimulate cellular metabolism and further aid with the uptake of minerals.

For example, some amino acids stimulate root cells to open up calcium ion channels, allowing calcium ions to be taken up thousands of times faster than simple osmosis. Calcium helps seedlings develop thicker stems and stronger cell walls. Plus, calcium activates the enzymes that pump the growth hormones to the growing tips.

Read also: Rooting Hormones Organic Stimulants

Microbial Inoculants for Seedlings

Treating seedlings with microbial inoculants is also a good idea. Some microbes fix nitrogen from the air and feed it to the plant, others solubilize phosphorus, and some produce plant protection agents and rooting hormones. For example, some micro-organisms produce the amino acid tryptophan on the surface of the roots. The root cells easily absorb the tryptophan and transport it to the leaves, where it is converted into IAA. The IAA is then pumped to the roots to stimulate the growth of new root hairs.

Beneficial microbes also produce siderophores. The word siderophore literally means “iron carrier.” One of the strongest chelators of iron in nature, siderophores act indirectly as plant protection agents. As beneficial bacteria colonize the root surface, siderophores sequester iron and make it unavailable to foreign invaders so the roots are protected from root pathogens such as pythium and rhizoctonia. Some soilless mixes take advantage of the process by adding certain species of bacillus subtilis to the blend and calling it a biofungicide.

Propagation is easy if you have the right tools and follow a few simple instructions. Just give the seeds a good soaking, and provide adequate light and warmth. Afterwards, it only takes a little coaxing to grow strong, healthy seedlings with thick stems and dark green foliage. Propagation is the least expensive part of indoor growing, so don’t be afraid to throw in a few extra seeds, and always select the best of the best!

Read next: When and How to Transplant Seedlings into the Garden


Share This Article

  • Facebook
  • LinkedIn
  • Twitter

Written by Harley Smith

Profile Picture of Harley Smith
Harley Smith is the director of research for NPK Industries. A veteran in the hydroponics industry, Harley has more than 18 years of consulting and educating experience. He is regarded as an expert on plant nutrition and organic bio-stimulants, performing research and new product development in the US and Europe.

Related Articles

Go back to top
Maximum Yield Logo

You must be 19 years of age or older to enter this site.

Please confirm your date of birth:

This feature requires cookies to be enabled