Half of every tree exists underground. The root spread equals the space occupied by the canopy.

While people focus on the form, branches, leaves, flowers and fruits because we share the same above-ground world of those parts, that does not mean they are more important than the parts we cannot see.

“The community underground is far more diverse than the one above ground,” Bill Cullina, executive director of Coastal Maine Botanical Gardens in Boothbay said last month in his keynote address at the Grow Maine Green Expo in Augusta. (The expo is a trade show held jointly by the Maine Landscape and Nursery Association and the Maine Arborists Association.) He was speaking on “Sugar, Sex, and Poison: Shocking Plant Secrets Caught on Camera.” Despite the obvious attractions of sugar and sex, it was Cullina’s discussion of the underground world that I found most fascinating. I suspect Cullina might think the same way. In an aside, he said that the interpretive theme at the botanical garden this year is “Roots: The Other Half of the Story.” The staff is creating the programs this winter, which will begin when the gardens reopen for the season on April 15.

In his talk, Cullina explained how plant roots have relationships with fungi that help bring nutrition (or sugar) to the plants – the bit of science that gets some gardeners to add mycorrhizal fungi to the soil. Fungi also served as an essential link to making the world what it is today. Most people know that life began in the oceans and later moved onto the soil. But how? “Plants couldn’t transfer to the ground until fungal relationship developed,” Cullina said.

Without fungi, all life would still be in the oceans, where nutrients are dissolved particles that plants such as seaweed can absorb easily. Nutrients in the soil are more complex, and have to be broken down before the plants can use them. Fungi and other microscopic living things play a large part in the process.

Soil serves as the stomach for plants, Cullina said. While animals store the nutrients they need in their stomachs, trees use organic matter that is in the nearby soil as their food. That is why adding compost to the soil is so good for plants. That said, not all compost is created equal.

“When you buy bagged compost, it is not great compost to use, because all the living parts that help it break down are smothered in the plastic bag,” Cullina said. He prefers compost that is steaming, a sign that it contains many living things that are continuing to break down the wood and other carbon parts that make up the compost. He gets such compost delivered by the truckload.

But trees don’t merely take nutrients. They also give nutrients away to their neighbors. This process is explained in “The Hidden Life of Trees” by Peter Wohlleben. Cullina expanded on the information in that book. He said that a huge hub or mother tree in a forest has many leaves and reaches high in the sky, so it creates a lot of food that it can send to nearby trees, which are likely its offspring. Understanding how that process works, Cullina said, brings into question the practice of loggers who harvest all the big trees and leave the small ones. He said it might be wiser to let the largest trees stand.

But it isn’t only their own children, or even their own species, with which trees share nutrients. In forests where birches and Douglas firs are dominant species, the birches send nutrients to the firs in summer when they have leaves and create excess food, and the firs send nutrients to the birches in winter, when the birches have lost their leaves, Cullina said. Humans could take a page from that type of selflessness, I’d say.

Enough about my obsession with the underground world. Let’s move on to sugar and sex, which almost everybody enjoys.

Cullina said his job – and the job of others who grow things – is to be “a manager in a photosynthesis factory.” Photosynthesis occurs in plants when sunlight converts water, made up of hydrogen and oxygen, and carbon dioxide, made up hydrogen and oxygen, into sugar, made up of carbon, hydrogen and oxygen. All that sugar made by photosynthesis keeps plants alive, and the more sugar, the better the show. As an example, Cullina showed how crested iris in their natural habitat have sparse flowers, but with good compost in good soil at the botanical garden, they produce triple the number of blossoms.

“Sex in plants is very G-rated,” Cullina said. Because plants can’t travel to meet their potential mates, they have to get others to carry the male pollen to the female plants for fertilization. Plants thus produce flowers that create nectar, look like they contain even more nectar, release heat or smell like rotting meat to attract bees, butterflies, flies, other insects and birds, who then transport the pollen for them.

Such clever creatures these trees and flower-producing plants.

Tom Atwell is a freelance writer gardening in Cape Elizabeth. He can be contacted at:

tomatwell@me.com