We all know that sugar isn’t the healthiest food for people, but for plants and the microorganisms that support them it can be a valuable source of energy and nutrients, boosting both the quantity and quality of your harvest. We’re not talking about ordinary table sugar here, but rather molasses, a by-product of sugar production. Before you grab a bottle of molasses from the shelf at your local supermarket and pour it on your crop, here’s what you need to know about where it comes from, how it works, and how to use it.
When sugar cane or sugar beets are processed, the pure sucrose is extracted, leaving behind highly viscous syrup. After that fluid is boiled, it becomes what’s called “Barbados” or “mild” molasses. Very sweet and light in color, it’s used to flavor hot cereal, tea, and other foods. A second boil yields dark molasses, an amber-colored syrup that’s commonly used in cooking and baking. When boiled a third time, much of the sugar is gone and what’s left is dark, strong-flavored – almost bitter-tasting – blackstrap molasses. It is used primarily in cattle feed and it’s also a key ingredient in dark rum.
The repeated boiling concentrates nutrients, making blackstrap molasses a rich source of calcium, magnesium, potassium and iron. Because of the high mineral content, health-conscious eaters use blackstrap molasses as a supplement in their diets.
Sulfur is a naturally occurring mineral in blackstrap molasses, but manufacturers often add sulfur dioxide as a preservative that blocks microbes from reproducing, keeping the syrup from spoiling too quickly. While the sulfur dioxide may be a benefit to consumers, for reasons we’ll explain the antimicrobial effect is counter-productive when using molasses to feed plants.
Plants produce their own sugars through photosynthesis and use it as fuel for cell growth and development. Sugar is also secreted through the roots to activate microbes that help break down nutrients and make them easier for plants to take up. Adding sugar, in the form of molasses, to the feeding program for your indoor garden sets the beneficial microbes on a feeding frenzy, which leads to a dramatic increase in their population.
The key minerals that make molasses a healthy food for people are vital for plants, too. Potassium and calcium, in particular, play an essential role in the processes that plants go through as they form buds and flowers. Even better, the sugar works as a natural “chelating” agent, binding it to other nutrients so they are more readily absorbed by plants.
As plants mature and prepare to bloom, their need for carbohydrates outpaces their capacity to produce them. Molasses is useful to plants throughout their life cycle, but it is most valuable at the transition from the vegetative stage to the peak of bloom time, when the need for sugars peaks. The extra carbohydrates in molasses give plants a boost that helps them flower more abundantly than if they relied solely on self-made sugar.
Incorporating molasses into your regular feeding cycle is most effective and easiest when it’s combined with other natural ingredients in well-balanced liquid fertilizers. All of the nutrients in the solution are more readily absorbed when molasses is in the mix and the sugars are available exactly when plants need them most.
If you want to try a more DIY approach, you can buy blackstrap molasses – just be sure to get the unsulfured type so it doesn’t kill off the very microbes you want to feed. Add it to your nutrient solution at a rate of about 1/4 cup per gallon. While you may hear recommendations about spraying molasses solution on plants directly, the residue can attract fungus gnats and other pests to your grow room. Also, the sticky substance is prone to clogging sprayers.
You get the most benefit from molasses in your nutrient solution in soil-based systems, though it works for hydro, too. If adding your own molasses, be sure to check the pH of the solution before giving it to your plants – the sugars will pull it toward the acidic side.
When your plants are two to three weeks from the end of their growing cycle, stop giving them molasses and other fertilizers and give them only water to “flush” out any unused nutrients. If you have leftover blackstrap molasses, you can mix it up at a rate of about a cup per gallon and pour it on your compost pile, where it will stimulate the good microbes at work there, too.
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