Picture two pots of basil side by side on a windowsill. The seeds are the same and went in the same day. One plant is bushy and full, while the other stopped growing after a few leaves. Both got exactly the same amount of water. What usually makes the difference is the nutrients their roots were able to take up.
One study of hydroponically grown basil showed this gap very clearly. Researchers compared plants given nothing but purified water with plants given a proper nutrient solution. With the solution, the broad-leaved 'Lettuce Leaf' variety yielded about 43 times more leaves, and the deep-purple 'Purple Opal' yielded about 60 times more. On water alone, the basil was barely hanging on.
Water Alone Won't Grow Basil
We grow basil for its leaves, and to keep putting out new ones it needs a steady supply of nitrogen and other nutrients. Give it only water and it can live for a short while on what's stored in the seed, but it has nothing to build new leaves from. In the study, the biggest harvests came from plants fed a mineral nutrient solution and from plants fed a solution with added organic matter.
You could see the difference in leaf color, too. The better a plant was fed, the deeper green its leaves were, because they held more chlorophyll, the pigment plants use for photosynthesis. Darker leaves capture more sunlight, and the plant uses that energy to grow still more leaves. Of the two varieties, 'Purple Opal' built up the most chlorophyll.
Each Variety Feeds Differently
Even on the same solution, the two varieties didn't perform the same. 'Purple Opal' grew more leaves than 'Lettuce Leaf,' and its leaves were denser and higher in vitamin C. The varieties also responded best to different solutions, so the one that produced the biggest harvest for one variety wasn't the best for the other.
So when you pick basil for your pots, think about the variety and how you'll feed it at the same time. If you want fragrance and color, choose a purple-tinged variety. If you plan to cook with a lot of it, choose one with big leaves.
Better Aroma and Nutrition Too
Well-fed basil didn't just grow bigger leaves. It also built up more of the aromatic compounds behind basil's familiar scent, along with more vitamin C. The well-fed plants had both a stronger fragrance and sturdier leaves. Those same compounds are what keep the aroma around after you add basil to a dish.
This is easy to try at home.
- Don't give plain water alone. Use a nutrient solution made for leafy greens, mixed weaker than the label recommends.
- Weak, frequent feedings are easier on the roots than one strong dose.
- Choose a purple variety for fragrance and color, or a large-leaved variety for cooking.
If your windowsill basil has been getting nothing but clean water, add a weak nutrient solution starting with your next watering. In a couple of weeks, you'll notice bigger, more fragrant leaves the next time you pinch one off.
