Molasses

Adding sugary, nutrient-rich molasses to gardens and compost piles is said to feed important microorganisms, raise sugar levels in your plants, and kill pests.

Truth or myth? Let’s find out.

Molasses (Marshall) CC BY 2.0

What is molasses?

As a young child, I thought nothing tasted better than freshly baked bread that had been slathered with butter and molasses. I still love it. But what is molasses and why do people say it’s good for your garden?

Molasses is the sweet, syrupy residue left over after processing sugar cane and sugar beets to make sugar. Also known as black treacle (UK), molasses is mostly sugar. It also contains calcium, iron, magnesium, manganese, phosphorus, potassium, sodium, and a little zinc.

Does molasses feed microorganisms?

The microorganisms that support our plants love sugar and all those minerals. Adding molasses to compost piles and planting beds makes sense. Provide food for microorganisms and they will multiply, doubling their population every 20 minutes, for as long as the food supply lasts. Therein lies part of the problem.

Natural processes take eons to find balances that allow them to continue. The variety of bacteria and other microorganisms responsible for decomposition, nutrient transfer, and healthy soil exist in a dynamic dance of available resources. Population explosions rarely end well and the same is true for all those microorganisms. As soon as the molasses is gone, they die.

Your plants will get some nutrients from all those dead organisms. And your compost pile will cook faster until you wet it down tomorrow and wash the sugars away. But those potential benefits are short-lived and have not been demonstrated in lab or greenhouse tests. Yeasts tend to grow faster than the other organisms when fed molasses, which may not necessarily be what you want. To maintain healthy microorganism populations, you are better off adding healthy foods, such as wood chips, compost, and other organic matter, rather than the junk food rush of molasses.

Can molasses boost plant sugar levels?

Proponents of molasses in the garden urge you to spray molasses all over your plants, swearing that you “can’t overdo it” and that the sugar, or Brix, levels of your plants will skyrocket, giving them abundant simple sugars that will carry them through lean times. First, simple sugars contain too much water and are too reactive to be stored. Second, the majority of that sugar would never get through the plants’ waxy cuticles. The minuscule amount of sugar that might be absorbed through the stomata wouldn’t make a significant difference in sugar levels. Brix levels are a function of overall plant health and photosynthesis, not exterior coatings.

Garden sprayer (Kate Russell)

Can molasses kill insects?

If you pour molasses on a soft-bodied insect, it will die. This doesn’t happen because molasses contains some magical property or the pest’s supposed inability to process sugar. The insect would simply suffocate. And saying that eating sugar will kill an insect is ridiculous. Sap is sugar water. Beetles and cockroaches eat candy bars and cookies. When sugar is eaten by insects, they do not explode. They fart like everyone else. [Did you know that termite farts are responsible for generating 20 million metric tons of methane each year? I didn’t either.]

​So, there you have it. Molasses can briefly boost microorganism populations. It will not significantly increase sugar levels. And it is not an insecticide. Rather than pouring it on your garden, save molasses for your toast. Or, try your hand at George Washington’s recipe for small beer (below).

George Washington’s small beer recipe

Take a large Siffer [Sifter] full of Bran Hops to your Taste. Boil these 3 hours. Then strain out 30 Gall[ons] into a cooler, put in 3 Gall[ons] Molasses while the Beer is Scalding hot or rather draw the Molasses into the cooler & St[r]ain the Beer on it while boiling Hot. Let this stand till it is little more than Blood warm. Then put in a quart of Yea[s]t if the Weather is very Cold, cover it over with a Blank[et] & let it Work in the Cooler 24 hours then put it into the Cask — leave the bung open till it is almost don[e] Working — Bottle it that Week it was Brewed. — From a notebook (c. 1757) kept by George Washington.


For another fun read about George Washington, check out my friend's new book about what happens when George gets transported through time in Finding George Washington.