How sweet is walnut sap? Two answers
Black walnut (Juglans nigra) makes a sweet sap that boils into a syrup Cornell describes as surprisingly similar to maple, with a nutty note. How sweet the sap is depends on who measured it, and the two research programs that have disagree.
| Who measured | Spring sap sugar | Sap per gallon of syrup (Rule of 86) |
|---|---|---|
| Cornell Maple Program, 2014 trials (4 sites in NY, PA and IN; 58, 96, 35 and 10 trees) | 2–3.5%, many trees 4–5%, one reading 6.2% | about 25–43 gal |
| Future Generations University, West Virginia (reported 2021) | 1.0–1.5 °Brix | about 57–86 gal |
Cornell's Michael Farrell wrote that the sugar content of walnuts mirrors that of maples, and that fall sap, by contrast, runs under 1%. The West Virginia team's reading is half that or less. Neither has been reconciled by a third study we could find, so measure your own trees with a sap refractometer and run the number through the sap to syrup calculator. The black walnut tree page carries the Cornell range.
The real limit: volume, not sweetness
Even at maple-like sugar, walnuts give far less sap. Cornell's sites made 6 to 11 ounces of syrup per tap, where maples usually give 32 to 64 ounces. Future Generations University's first season (107 taps, 2019) averaged 1.6 gallons of sap per tap; a bucket tap on a sugar maple averages about 10. In its 2020 and 2021 trials, low vacuum nearly doubled sap output, but higher vacuum did not add more, unlike maple.
Set expectations by the cup, not the gallon
When to tap: with the maples
Walnut sap appears to run around the same time as maple sap, according to UNH researchers quoted in a 2021 Vermont Public report, and Cornell tapped most of its trial trees in mid-February. One Indiana cooperator tapped on November 19 to test autumn flow; walnuts can run in fall, but that sap is under 1% sugar. Cornell's advice, reflected on our tree page, is to tap early, since sugar may decline through the season. The sap flow forecast is built on maple's freeze-thaw rule, which is the same signal you are waiting for here.
Tapping a black walnut
The basics are the maple ones in how to tap a maple tree, with two walnut-specific notes from the West Virginia work:
- 1
Be sure of the tree
Walnut is alternate-branched with big three-lobed leaf scars and a chambered pith; butternut looks similar and, per Future Generations University, has similar characteristics and can produce syrup. The black walnut tree page has the winter ID and the butternut comparison. - 2
Pick young, fast-growing trees
In a 2006 Kansas State study, as summarized by Future Generations University, sapwood thickness was the best predictor of sap volume, which favors young, vigorous trees over old ones. - 3
Set the spout gently
Walnut is semi-ring-porous with softer wood than maple. Do not over-drive spouts. - 4
Tap when the maples run
Mid-February was Cornell's timing; the freeze-thaw signal is the same as maple's.
Pectin, filtering and walnut jelly
The thing nobody warns you about is pectin. Walnut sap carries it, and Future Generations University says it can gum up a reverse-osmosis membrane, inhibit filtering, and make walnut syrup into walnut jelly. Cornell's Farrell put it as pectin making filtering the sap and syrup extremely difficult: it clogs filters much faster than maple's sugar sand, and it varies by tree, site and time of season.
Plan on more filter changes than a maple batch of the same size, and expect a syrup that may set up thicker than you intended. The sources we read do not give a walnut-specific filtering fix, so we are not inventing one.
Where walnut syrup is made
The documented production is research-scale and in the eastern hardwood belt: Cornell's cooperators in New York, Pennsylvania and Indiana; Future Generations University's trials in West Virginia; and, per Farrell, Japanese walnut (the heartnut species) grafted on black walnut tapped in Ontario by Todd Leuty of the provincial agriculture ministry. Farrell also notes butternut was studied in Michigan in the 1920s with promising results in comparison with maples, and that English walnut sap flow has been studied in France, though not for syrup.
Every species of walnut produces a sweet sap that can be boiled into syrup, Farrell writes. Who sells it, and where, is not something the research we relied on documents, so look to local producers and festivals rather than this page for that.
