Then he went into the maple woods and with the bit he bored a hole in each maple tree, and he hammered the round end of the little trough into the hole, and he set a cedar bucket on the ground under the flat end.
The sap, you know, is the blood of a tree. It comes up from the roots, when warm weather begins in the spring, and it goes to the very tip of each branch and twig, to make the green leaves grow.
-Little House in the Big Woods
Laura Ingalls’ Little House series was my favorite series growing up. The most fascinating chapters for me were the ones describing how Laura and her family made food without having to go to a store. Almost everything they ate was homemade, including their maple syrup and brown sugar. Every winter, Laura Ingalls’ grandpa would collect maple tree sap in big cedar buckets. It was an affair that the whole family participated in, and Laura learned the process from a very young age.
As Laura's father poetically described it, sap is the blood of a tree. It contains sugar, nutrients, and other chemical compounds that nourish the tree. The “vessels” tasked with transporting the sap throughout every branch of the tree are called xylem and phloem, plant tissues found in all vascular plants.
Phloem is a living tissue that transports food made from the leaves by photosynthesis to other parts of the plant. Xylem is a rigid, dead tissue, and it is what we refer to as “the wood” of a plant. The xylem transports water collected by the roots up the stem and into the leaves. Maple syrup is an example of xylem sap.
Sap will not flow out of the tree any time of the year; it depends on two concepts: temperature and pressure. Sap is typically harvested in late winter and early spring during “sugaring season,” when temperatures fluctuate below and above freezing day and night. This thawing and refreezing cycle is what builds pressure inside of the trees. Thawing during the day creates positive pressure in the tree (like a swollen balloon), pushing sap out of any tap holes. Refreezing at night creates negative pressure, pulling ground water back into the tree. Now that the tree’s water is replenished, it can continue the flow of sap the next day.
If you had dreams of drinking maple syrup straight from the tree, you will likely find yourself disappointed. Only about 2% to 2.5% of sap is sugar. To make thick and sweet maple syrup, much of the sap’s water content must be removed. In Laura Ingalls’ time, they boiled the sap over large bonfires to evaporate the water. Nowadays, factories put sap through a reverse osmosis machine before boiling. The reverse osmosis process is similar to wringing out excess water from a towel before throwing it into a drying machine; it passes sap through a very, very small sieve that only allows water molecules to pass through. About 43 gallons of watery sap creates 1 gallon of thick, sweet maple syrup. It is this finished product that we can joyously douse over hot pancakes, just as Laura Ingalls did 150 years ago.
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