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Tapping Walnut Trees: Studies on Walnut Sap Flow

Tapping tree species other than sugar maple provides syrup producers the opportunity to apply their skills and use their equipment to expand into new and potentially lucrative markets. Just as bourbon barrel-aged syrup and various flavor-infused syrups are opening new markets for maple syrup, the unique tastes of alternative tree syrups, and their maple blends, are finding a place in today’s “foodie” economy.

In North America, Black Walnut (Juglans nigra) is the most tapped species besides sugar maple (Acer saccharum). When considering tapping, however, it is good to understand walnut trees are not just maples with compound leaves and big edible nuts. Walnuts have anatomical and physiological characteristics that affect both tapping and syrup making.

Walnut – A Different Tree

Wood anatomists classify maple as a diffuse-porous hardwood. In maple trees, small vessels, commonly called pores, carry water and nutrients up the tree in the summer and carry sweet sap to your tap during the sugaring season. These vessels are evenly distributed or diffused throughout the tree’s annual growth ring. Walnut trees, on the other hand, are classified as semi-ring porous species. They have small diffused pores like maple, but also large pores – similar to a true ring-porous species such as oak and hickory – that are more prominent in the early annual growth. As anyone who has ever looked at the end of a walnut log can tell you, walnut trees have a large, dark heartwood area, with a small band of white sapwood. A young healthy maple tree, on the other hand, can have mostly sapwood. Maple, and especially sugar maple, also called “hard maple” or “rock maple,” is a very hard wood, whereas walnut wood is comparatively soft.

Finally, walnut sap, with a brix of 1.0 to 1.5, also contains pectin. Pectin is a natural constituent of plant cell walls where it helps bind adjacent cell walls together. And, if you’ve made fruit preserves, you can attest, when boiled in an acid environment, pectin forms a gel. When present in high enough concentrations this pectin can gum up an RO (reverse osmosis system), inhibit filtering, and turn your walnut syrup into walnut jelly.

Tapping Walnut Trees:

People have been tapping trees and making maple syrup for a long time. Indigenous people living in northeastern North America were the first groups known to have produced maple syrup and maple sugar, long before European settlers. Today, there is an extensive body of research that has led to high efficient practices and increased production. The same is not true for walnut. With walnut trees, there are just a few studies out there all conducted within the past twenty years.

For the past three years, Future Generations University’s (FGU) Appalachian program has been studying various aspects of walnut sap flow and syrup production. This article presents a synopsis of the results of that work and relates those findings to the unique anatomical and physiological characteristics of walnut.

The 2019 Season: During the 2019 field season we put 107 taps into walnut trees, about half on sap bags and half on 3/16 tubing. This was our first year of tapping walnut and it was a season of trial, error, and observation; mostly error. The average sap collected per tap was 1.6 gallons. Certainly, nothing to write home about, especially for a sugar maple sap collector. Even though we had over 20 feet of elevation change on the 3/16 lines, we were never able to develop more than 9 inches Hg of natural vacuum. Theoretically, we should have been closer to 18 inches. We noted that the straight barreled polycarbonate spouts were seated deeply in the soft walnut wood, potentially cutting off sap flow (see Rechlin, 2019, mapleresearch.org). The deep spout seating combined with the thick walnut bark meant that many spouts were buried up to the shoulder. We also noticed that the 3/16 tubes were mostly filled with gases, whereas maple sap collection tubes are mostly filled with sap. By analyzing a series of photos, we determined that walnut tubes contained only 9% sap compared to 85% sap in maple. This lack of a continuous sap column was most likely why we failed to develop any appreciable vacuum.

7/16-inch spout

7/16-inch spout and a longer barreled walnut spout

The 2020 Season: In 2020, we worked with the Robert C. Byrd Institute for Advanced Manufacturing to develop a longer barreled, more highly tapered spout for walnut so it would sit at less of a depth, (Figure 1). Working with 4 walnut syrup producers collecting with buckets we found that the spouts they were using produced an average of 1.7 gallons of sap/tap whereas the new walnut spouts produced 2.6 gallons of sap/tap.

That same year we also reached back to the Ferrell and Mudge study from 2014 and tried our hand at applying artificial vacuum to our lines. We established 2 lines with 20 taps on each line on trees in a similar streamside environment. Without vacuum, the A line out produced the B line in 3 of the 5 collections, with both lines producing equal amounts of sap in the other two collections. A Shurflo DC diaphragm pump was then installed on the lower producing B line. With an average of 8 inches of vacuum the B line produced 0.74 qt/hr., double the sap production of the A line with 0.37 qt/ hr., (Figure 2).

This brings us to this past sap flow season, 2021. Having shown that a low level of vacuum significantly increases sap production, the next question is what could you achieve with higher levels of vacuum?

The 2021 Season: This past season we attempted to answer that question. The walnut syrup producer we were working with installed a vacuum pump allowing us to regulate the amount of vacuum on each of our research lines. Figure 3 shows the amount of sap collected in vacuum chambers with each line at 15 inches Hg vacuum. As in the previous season with gravity flow, when both lines received the same level of vacuum, the A line, producing 1.4 qt/hr., outproduced the B line which averaged 0.9 qt/hr. From this, we can gather that the A line trees are intrinsically more productive than the B line trees.

We then dropped the vacuum to zero (gravity flow only) on the A line and applied 22.5 inches of Hg vacuum, the maximum this system would provide, to the B line (Figure 4). The B line trees now produced 0.5 qt/hr., outproducing the A line at 0.3 qt/hr. As with the previous year, the application of vacuum increased sap flow.

Figure 4. Sap collected in qt/hr. without vacuum on the A line and 22.5 inches of vacuum on the B line.

Next, we raised the vacuum on the A line to 10 inches, which reduced the maximum achievable vacuum on the B line to only 17 inches. With maple, a higher vacuum positively correlates to greater sap flow. However, here the relatively lower vacuum level A line averaged 0.4 qt/hr., whereas the higher vacuum level B line produced only 0.2 qt/hr. Again, March 11th was an anomaly, with B outproducing A (Figure 5).

Interesting, but what might be happening and how does it relate to the anatomical characteristics of walnut? Diffuse porous hardwoods, like maple, create stem pressure in the spring to dissolve gasses in their xylem allowing growing season sap to flow up the tree to support bud break. Ring porous species followed a different evolutionary strategy. They just grow large new vessels in the spring to transport water and nutrients, not bothering to develop pressure to dissolve the gases in their older vessels. Which, by the way, is why you should not bother tapping an oak or ash tree. Walnut, being semi-ring porous, is somewhere in the middle. They do create pressure to clean out their small vessels, but that may not apply to their large vessels, which then would remain filled with gas. This could account for the large quantity of gas we recorded in 2019 as present in the walnut sap. It could also explain why, in this study, more vacuum did not lead to more sap flow, just the pulling of more gas from those large vessels.

I liken it to trying to get those last drops off the bottom of your milkshake. You can suck really hard on a straw and just swallow a lot of air or reduce the vacuum you are applying to the straw and finish off the shake.

Summary of Walnut Tapping – Lessons Learned

1. With walnut’s softwood it is important not to overdrive the spouts. The familiar hammer bounce and pitch rise on a straight barreled maple spout is too far. Choose a spout with as much taper as possible. Hopefully, we will soon have a commercially available walnut-specific spout.

2. Vacuum does increase sap flow. Relatively low levels of vacuum nearly doubled sap output in the 2020 and 2021 studies.

3. Unlike what is expected with maple, in this study, higher levels of vacuum in walnut did not result in corresponding increases in sap flow. However, it should be stated that as a preliminary study this result should be corroborated in future research.

FGU’s Appalachian program continues to research and promote the tapping of alternative tree species. In this coming season, we will be working with the Byrd Institute to develop and commercialize a more productive spout for tappable species with a softer wood, and with Marshall University on the pectin issue. Our work this coming season will also include the expansion of preliminary studies we have been doing on tapping and syrup production from American sycamore (Paltanus occidentalis). Whether in addition to tapping maple or as an expansion of tree syrup production beyond the commercial sugaring range of maple, the unique flavors of alternative tree syrups are helping expand our industry and serving niche markets for the sweet flavors from our forests.

Acknowledgements: This work was supported through a WVDA specialty crop block grant, NE SARE Partnership grant ONE19-374, and by the Claude Worthington Benedum Foundation.

Citations:
Ewers, Frank, Thierry Ameglio, Herve Cochard, Francquis Beaujard, Michel Martignac, Marc Vandame, Christian Bodet and Pierre Cruiziat. Seasonal variation of xylem pressure of walnut trees: root and stem pressures. Tree Physiology (21) 2001.
Ferrell, Michael and Ken Mudge. Producing Maple Syrup from Black Walnut Trees in the Eastern United States. Maple Syrup Digest. December 2014.

Gross Influences on Heartwood Formation in Black Walnut and Cherry. USDA Forest Service. FPL 268. Forest Products Research Lab. Madison. WI. 1976.

Naughton, G.G., W.A. Geyer and E. Chambers IV. Making Syrup from Walnut Sap. Transactions of the Kansas Academy of Science. Vol 109, no.3/4. 2006.