You get tall, spindly or leggy basil seedlings when your soil is too nutrient rich, there is not enough light, you fail to prune your plants or the temperature is too hot.
And it is often a combination of at least two factors where not providing enough light is almost always the main culprit.
The good news is however that basil is one of those herbs where leggy seedlings can be used to propagate more plants.
Why we welcome leggy basil seedlings
Basil is one of our favourite herbs to grow as we like to use pesto in our cooking. And you need a lot of basil to make pesto.
Our basil seedlings often grow leggy when we plant our seeds between the months of October and March when levels of natural light are low to, at times, almost non-existent.
But we have devised a method that works very well for us. We grow basil from seeds in starter pots and place the pots under some – but not enough light.
We may for example put the starter pots in the kitchen on a surface area that is lit during the day.
The result will always be well developed but leggy seedlings. And this is where it gets interesting.
We will consistently double the number of plants we grow by for example turning 4 leggy seedlings into 8 basil plants. And for us this means more pesto.
How to grow more basil from leggy basil seedlings
1. Start with leggy basil seedlings that are at least 15 cm (6 inches) tall
Use a sharp pair of scissors to cut the stem just above where you can see new sets of leaves forming.
2. Cut all leaves from the stem cutting leaving only the top pair of leaves
If the cutting is shorter than 10 cm (4 inches) we leave at least one set of fully formed leaves.
3. Place basil cuttings in a glass or container of water
If you have the space, use one container for each individual cutting. Replace water daily and always use room temperature water.
4. Transplant remaining seedlings from starter pot
Next, transplant seedlings from starter pot into a larger pot with fertile soil that drains well. Keep soil moist and place it in a spot with good light.
Related reading: Read our articles about transplanting tomatoes and peppers in the articles When to pot up or transplant peppers (and how)? and Transplanting tomatoes: How to transplant tomato seedlings
5. If you started with 1 leggy basil seedling you now have 2 separate basil plants growing
You have 1 basil cutting placed in a glass container and one basil seedling growing in a pot with fertile soil for each of your leggy basil seedlings.
6. After 1-4 weeks you will see fully formed white roots growing from your basil cuttings
7. Transplant each basil cutting into a pot with fertile soil that drains well and place in a location with plenty of light
And there you have it, you have turned each individual leggy basil seedling into 2 separate basil plants.
If we had an unlimited number of grow lights we would most likely grow all basil seedlings in perfectly lit conditions. But, as we only have a limited number of grow lights, we have found this method to be a great way to consistently and quickly grow more basil plants year-round.
4 reasons basil plants get spindly or leggy
We have already listed four factors that will cause your basil plants to grow tall, leggy and spindly.
But to avoid growing leggy basil seedlings and plants in the future, you must understand why basil grows spindly and leggy. Because it often starts already when you plant your seeds.
1. Soil is too nutrient rich for basil seeds and seedlings
When you plant your seeds it is important to use a lean potting soil or seed starting mix.
Because the seed itself holds all the energy needed for the seed to germinate and sprout first leaves.
Using soil that is nutrient rich can stress the plant to grow and develop too fast resulting in tall, weak and leggy plants.
Always use a lean potting soil or seed starting mix to give your seeds the best possible environment to germinate and sprout.
2. Not providing enough light for basil seedlings
Basil wants full sun for at least 6 hours per day to grow and thrive. And if you are not providing enough light, seedlings will use all their energy to grow tall in search for more light.
The stem will grow long and you end up with a top heavy plant that is unable to carry its own weight.
When starting basil seeds indoors in zone 7, there is not enough natural light in early spring. We always use artificial grow lights to help our seedlings develop into strong and compact plants.
3. Failing to prune basil plants
Growing herbs and plants from seed, it can feel counterproductive to prune seedlings and young plants that have taken weeks to get going.
But basil needs to be pruned to grow into a strong, compact and healthy plant.
We start pruning basil when our seedlings have a minimum of 3-4 sets of true leaves growing from the branch. For most varieties of basil including Thai basil your plant will be about 15 cm (6 inches) tall at this stage of development.
Inspect the stem and use a pair of sharp scissors to make a cut just above where you see new leaves forming. Be careful not to hurt or damage these new leaves.
Pruning your basil plant will give you more leaves to harvest and a healthier, bushier and more compact basil plant.
4. Growing environment too hot for seedlings
Mature basil plants will grow and thrive in hot temperatures up to 32 degrees Celsius (90 F).
But whereas basil seedlings want a warm environment to develop, it is my experience that too hot will stress basil seedlings to grow tall and leggy.
Aim for a temperature around 22-26 degrees Celsius (72-79 F) when germinating basil seeds. But as soon as the seeds have germinated and sprouted first leaves, lower the temperature to around 20-22 degrees Celsius (68-72 F).
For us, this method has proven to produce strong and compact basil plants.