Perennial flax: a new crop for sustainable agriculture in the Northern Plains

Objective 1: Agronomic recommendations for sowing, stand establishment and weed control.

Sowing of perennial flax (Linum lewisii) experiments 1 and 2 was planned in the fall of 2019, prior to the start of the grant funding, but was prevented by persistent wet conditions in August and September that not only prevented planting of fall plots, but also prevented producer harvest of spring wheat (Triticum aestivum), resulting in loss of many fields due to pre-harvest sprouting and decay in the wheat crop.

Sowing of experiments 1 and 2 was attempted in May of 2020 with success. Our Comstock, MN, location was planted 27 May 2020 and Carrington, ND, was planted 29 May 2020. Approximately 2 weeks after planting, as flax seedlings were emerging (Figure 2), a thunderstorm with heavy rain caused soil movement that buried newly emerged seedlings at our Comstock site, resulting in nearly complete loss of stand. Slow emergence of the seed was not due to seed-related low vigor, as the seeds uniformly and quickly germinated in the laboratory. Rather, the species seems to emerge slowly in the field, for reasons that need to be investigated, but seems to be a "conservative" approach that focuses on root development and access to moisture before emerging, based on the large root biomass of young plants. The Carrington site planting received inadequate rainfall after planting, which resulted in poor development of the flax and many of the weed management companion crops. However, the oat and field pea mixture for Experiment 1 did emerge nicely and provide some control of crop mimic weeds (common lambsquarters and Amaranthus spp.) that showed up later when rains arrived. Later emergence of perennial flax, with these rains, resulted in a poor-to-fair stands, because we observed that the perennial flax was somewhat content with having fast growing annual cover, as long as it was not too dense (Figure 3). 

The Comstock, MN, site was replanted for both Experiment 1 and Experiment 2 on June 24. During the following 2 week period, another heavy rain event occurred and resulted in washing/crusting of soil, which resulted in almost no plant stand for both the perennial flax and also many of the intercrops in Experiment 2. Sowing depth was between 0.5 and 0.75 inches deep for all the plantings to this point. We hypothesized that sowing depth, combined with poor soil conditions due to crusting, was preventing stand establishment, and that, perhaps, having a nurse crop in the same planting furrow may help emergence when crusts develop. We decided to investigate this further with another experiment at Prosper, ND (near Fargo), which we planted on August 6, 2020. The study design incorporated a factorial combination of the following treatments: 4 seeding densities of perennial flax (2x, 4x, 6x, and 8x the seed production rate on the NRCS guide sheet), 4 intercrops (none, annual flax, natto soybean, and hard red spring wheat), and 5 planting treatments (surface-no packing, surface-packed, 1/3" deep, 2/3" deep, and 1" deep). In addition, half of each plot was left to heavy precipitation, which occurred naturally, while the other half had the soil loosened slightly after crusting with a Garden Weasel. This was an unreplicated preliminary experiment to determine if any of the treatments would show any signs of efficacy. The only treatments to result in more than a few seedlings emerging were the surface-seeded plots (both packed and unpacked; Figure 4; Table 3). There was no benefit of intercropping in-furrow for any of the depth treatments. The conclusion we reached is that perennial flax, in its current form, is very susceptible to heavy rain events that have become common from late spring through summer in the Northern Plains region. Seedling vigor is, however, a logical target for plant breeding to solve.

Our fall planting of the updated Experiment 1 (see updates in Methods) was conducted on 5 September 2020, after the spring wheat harvest window.  Although not seeded into spring wheat stubble, we simulated what this planting window would be like in order to understand winter survival issues, if they arise. Learning from previous experiments, we reduced the seed depth to 1/8-1/4" deep and planted oats as nurse crop perpendicular to the direction of planting of the perennial flax (as was planned in the proposal). Rates of seeding were also increased to 4x and 6x, instead of the 1x, 2x, and 4x seed production rates initially proposed. As heavy rainfall is not typical in fall, and did not occur, we had excellent emergence and stand establishment. Consistent with observations in Carrington, the perennial flax tolerated the oat cover well, and the oat established well enough to provide a "snow catch" to maintain winter cover (Figure 5). Experiment 1 was only planted at Comstock, MN, for fall sowing date because Carrington, ND, was still too droughty for emergence of shallow-seeded crops.  Because of the lack of success of spring/summer planting, due to commonly experienced heavy rain events, we added a treatment of "dormant seeding," which was planted 5 November 2020 at Comstock and 6 November 2020 at Absaraka, ND (in lieu of Carrington because of COVID related travel restrictions and drought; Figure 6). This followed the methodology originally outlined for Experiment 1, except planting depth was 1/8-1/4", planted into fall-planted oat companion crop, at 4x and 6x seed production recommended rate. The soil was cold at both sites at the time of planting, and prior to the first persistent snowfall, both fields were confirmed to have no sprouted seeds.

Figure 5. Fall planted (5 Sept. 2020) Lewis flax variety 'Maple Grove'. (L to R) 10/17/20, 03/27/21, 05/08/21, 05/30/21, and 07/11/21, Comstock, MN.

Figure 6. Dormant seeded (5 Nov. 2020) Lewis flax variety 'Maple Grove'. (left) emergence on 04/17/21, (right) progress on 05/30/21: dormant seeding left side, fall seeding right side, Comstock, MN.

As can be seen in Figures 5 and 6, fall and dormant seeding resulted in high emergence rates, almost zero overwinter death for fall seeding, very early  spring seed emergence (before weeds) for dormant seeding, and comparable stands and vigor for both planting schemes by early summer. Our assessment is planting in early fall, dormant seeding, or an early spring planting before heavy rains are likely to occur, are good options for establishing perennial flax seedlings. We recommend a small grain companion crop to minimize winter exposure for seedlings.

The results of the trial in 2022 was as follows. The perennial flax never fully canopied due to the typically low level of leaf material on the plants (Figure 7 - left and right). There was no lodging in any plots. Days to bloom were identical for all plots. Plant height was on average 6 inches taller at the 4X planting rate than the 6X planting rate (81.8 vs. 75.5 inches, respectively), with no significant differences based on row spacing or date of seeding. Flax seed yield and oil content on the mature stands in 2022 was similar between fall or dormant seeding timings. Flax yield and oil content was also not influenced by seeding density. Flax yield did not significantly differ between the 15" vs. 30" row spacings (51 vs. 57 lb/acre, respectively), but oil content of the seeds was significantly different (33.0 % oil in 15" rows vs. 32.2 % in 30" rows; F=15.5; p=0.0013). The average yield across all treatments was approximately 52.6 lb/acre, which is substantially below the average yield for annual flax which ranged from 672 to 1,595 lb/acre in North Dakota in 2020. However, we did have substantial losses due to flax boll shattering that left the seed on the soil between windrowing (Figure 7 - bottom) and combining. In our breeding work, we are already working towards more determinant flowering and reduction of boll shattering to increase harvestable yield (see below). We believe that eliminating this major problem is achievable with breeding in a short period of time. Low yield may also be due to lack of fertility; however, according to soil tests, residual N should have been adequate. Substantial weed pressure at both sites, being organic sites with heavy grass and broadleaf weed seed banks, likely reduced yield as well.  

Figure 7. (left and right) Extent of canopy cover from near-mature Maple Grove flax: 30" row spacing and 15" row spacing, respectively. (bottom) Windowing of perennial flax at the beginning of the first boll shatter (7 July 2022). Maple Grove is indeterminate, so this point was chosen for windrow to minimize seed loss and maximize mature seed set.

For Experiment 2, our initial failure to get excellent perennial companion crops established with Lewis flax led us to attempting other methods of weed control (see updated Methods). We first looked at flame weeding treatments on plants in the greenhouse. Three growth stages of perennial flax were flame-treated alongside a broadleaf (kochia, Bassia scoparia) and a grass (wild proso millet, Panicum miliaceum) weed species. The three growth stages of flax ranged in relative maturity with the youngest stage being seed germinating below the soil surface and oldest stage being flax with multiple true leaves. We noted good weed control in the greenhouse trial, especially among the broadleaf weeds. Perennial flax suffered high injury and mortality among the older two stages (75-85% mortality), while the youngest stage was minimally if at all affected by flame weeding (Figure 8). Older stage flax that survived flame weeding show a marked recovery response, initiating new growth within 7-14 days. Given these results, we determined that flame weeding in the field could potentially be effective if we could get the timing right. Additionally, we suspected that flax injured by flaming in the field may exhibit a stronger recovery response as they would not be limited in rooting depth and would have had a longer time to establish and recover.

Figure 8.  (Left subfigure) Lewis flax, wild Proso millet and Kochia (L to R) 14 days after a 1x flame weeding dose (front), untreated flax and weeds (right rear). (Right subfigure). Mean percentage mortality for flax treated with flame weeding at different growth stages in the greenhouse study. Lowercase letters denote mean separation (α=0.05).

The Absaraka field site was treated with flame weeding first on 18 May 2021. We treated the plots in the evening so as to avoid morning dew, which can reduce the efficacy of flame weeding. Unfortunately, the flax had emerged well before any weeds. This was the first time we had dormant seeded perennial flax, and we did not have a precise baseline for estimating emergence time. Flax was approximately 7-10 cm tall with multiple true leaves at the time of flaming. This was not ideal, as we experienced high mortality at this growth stage in the initial greenhouse trials. We still applied the treatments as we hypothesized that flax in the field may be more resilient to flame weeding injury, and that the result may differ from the greenhouse trial as we were using a more precise piece of equipment.

Following our initial flaming application at the Absaraka site, we noted 60-90% flax mortality (Figure 9). Unfortunately, we did not see lasting weed suppression from the initial flaming event. Existing weeds at the time of flaming were moderately reduced, however a second flush of new weeds emerged shortly after flaming, rendering the effort insignificant for thorough weed control. The intense drought conditions of summer 2021 likely compounded the injury to flax and impeded recovery. We determined that a second flaming application would likely totally eliminate flax from the field, so we decided to not flame at the Comstock site.

Given this unfortunate early result, we adjusted our approach to incorporate a new spread of flaming and tillage combination treatments on extra plots that had received no prior treatment. These additional plots were seeded at the same timing, spacing, and rate as the previously treated plots. We evaluated additional methods of mechanical weed management for these treatments, as well as scaling down our flame weeding to the use of a handheld light duty flamer for a more targeted application (Table 4). This second treatment scheme represents a more intensive, modified approach to using flame weeding with extra emphasis on reducing flax injury by reducing flax exposure to flame. These treatments were applied at both the Comstock and Absaraka sites during late August, after a flush of winter annual weeds (e.g., field pennycress) had emerged and established. Initial observational data supports good control of winter annual weeds and little to no flax mortality (Figure 10).

Despite the difficulties faced in the first year of utilizing flame weeding in perennial flax, we feel that there is still an optimal way to incorporate flame weeding as part of an integrated weed management plan for the crop. We believe that second year perennial flax may be more resilient to flaming injury. 

We also began establishment of a fall seeded plot of perennial flax inter-cropped with winter wheat to begin exploring this approach to weed management as well (see updated 2 in Methods). Emergence of fall seeded portions of the experiment were successful (Figure 11 - top). Winter wheat formed a competitive canopy that nearly eliminated annual weed pressure throughout the growing season and even postharvest (Figure 11 -bottom). Winter wheat mean yield differed between sites, with 36.8 bu/acre at Comstock and 89.5 bu/acre at Absaraka, indicating that the wheat may have been more competitive at Absaraka. Mean winter wheat yield did not differ between the full vs. 2/3 wheat seeding rates. Flax stand density did not differ between sites or winter wheat rates, but the 1X flax seeding rate produced less dense flax stands than the 2X rate (2.3 vs. 4.9 flax plants/meter, respectively), suggesting that increased seeding rates may be needed with a companion cropping strategy. At Absaraka, most of the established Lewis flax was found at the beginnings and ends of the planting rows, which indicates that the winter wheat competition may have been a little too intense, but at Comstock, both rates (i.e., 1X and 2X rates) emerged and established more uniform rows of Lewis flax (i.e., no edge effect) but the overall flax density was relatively low. Reducing the winter wheat seeding rate further, e.g., to a half-rate, could improve flax establishment but also increase the risk of competitive annual weed establishment. Although more work remains to test other potential winter annual or annual crops as companion crops that produce harvestable yields and inhibit weeds without significantly inhibiting perennial flax establishment, these early results suggest that such as strategy could be developed.

Figure 11. (top panel) Fall planted winter wheat and Lewis flax planted 24 Sept. 2021. Successful stand establishment as of date of photo (5 November 2021). (left panel) 'Noreen' winter wheat interseeded with 'Maple Grove' perennial flax, near winter wheat maturity (27 June 2022). Weedy check adjacent. (right panel) 'Maple Grove' perennial flax successfully established after winter wheat harvest, 2 November 2022).

Objective 2: Enhance our perennial flax breeding program by learning from producer experiences in participatory, on-farm trials

As a result of weather and Covid-related limitations to the research, we have condensed Experiment 3 and 4 into a common experiment. As stated previously, we had planned to conduct a fall nursery planting of perennial flax in 2019, but weather conditions that resulted in delays for our farmer-cooperator led us to abandon those plans in favor of starting greenhouse plants for Experiment 4 -- development of inbred progenies for breeding and evaluation. During the greenhouse work, in March of 2020, our efforts ran into two issues, the development of the Covid pandemic and the diagnosis of the breeding research specialist (funded in part by this grant) with cancer. The research specialist took a leave of absence while obtaining treatment (which was successful and he returned to work in 2022). In order to work through these issues, we went from a full plot evaluation approach to a spaced plant evaluation that also included production of self pollinations and crosses, essentially combining the objectives (Figure 12). We also developed a working relationship with Noreen Thomas, a farmer that is closer to Fargo, which allowed us to do more of the breeding work with less travel, and this allowed us to more easily be Covid-protocol compliant. Additional accessions have been added to the trial site in 2021, doubling the number of accessions we planned to assess (to 200) and doubling the geographic area of our collections to also include high elevation types from Colorado and New Mexico (Figure 13). Data indicates a wide variety in seed morphology, plant height, growth habit, flowering time and duration, and vigor among the accessions that were evaluated.  Efficient self-pollination protocols for the greenhouse have been developed, and progeny of these accessions are being grown in the greenhouse and field nurseries to continue the work of self-pollination to make uniform lines from these accessions for later release.

Figure 12. (L) Spaced plant nursery of perennial flax shortly after transplanting of seedlings. This was conducted on Doubting Thomas Farms, with participating farmer Noreen Thomas. (R) Brent Hulke and Noreen Thomas posing with a "picture frame" that we plan to set out in the field for visitors to take photos with the blooming perennial flax nursery.

Through our spaced plant nurseries, we have been able to identify wild collections and partially inbred lines with traits important for continued domestication. Autonomously self pollinating types have been discovered from Colorado and North Dakota plant collections. Types with determinate habit have been found in high elevation collections from Colorado.  Determinate habit is very important for ensuring that all the seeds mature at the same time, reducing shattering losses during harvest. Seed size differences are considerable between accessions, with larger seeded types arising from collections from the southern US. Larger seed size is a domestication trait that is associated with improved seedling vigor and increased oil content. Flower size variation is also considerable, from around 2 to 4 cm in diameter. Flower color variation includes white flowers and light blue flowers, with and without the presence of anthocyanin (red pigment) in the veins towards the center of the flower. While floral traits seem to be of minimal consequence in variety development, they can be quite useful for establishing distinctions between varieties, which may be useful for breeders in the future. The next steps for continued domestication are to develop knowledge of the inheritance of these traits and begin to put them together in a common variety, which will continue in the years after this project is completed. Even so, we are advancing lines with improvements over Maple Grove for near-term use and possible release.

Our agronomy experiments and previous science in domestication have indicated that increasing seed size in order to increase seedling vigor is possible and necessary to improve seedling emergence under challenging soil conditions like crusting. However, at the present time, planting very early or very late in the season also works for this hardy perennial plant. The extent of seedling tolerance to freezing conditions, however, is unknown. Graduate student Andre Gossweiler completed a freezing tolerance study in which 45 accessions from a cross-section of the native range (latitude, longitude, and elevation, Figure 14) were assessed for survival and vigor after a day-long deep freeze at several temperature points below freezing. Each plant was only exposed to one of the final freezing temperatures, and in doing so, we could see the response (in immediate death and also in survivor recovery vigor) of each accession as temperature treatments became colder. The resulting plants appeared similar to Figure 14-top two weeks after freezing. There were four accessions with 100% survival at the lowest exposed soil and air final temperature of 5 F. These included accessions from Alaska, western North Dakota, and very dry, high elevation conditions in Colorado. The lowest daily average soil temperature recorded by local weather stations in Fargo, ND, is 5 Fahrenheit, with lowest hourly of about 0 Fahrenheit. In the same experiment, the released variety Maple Grove ranked in the lower 50% for survival, despite overwintering very well as seedling and adult plants in the Fargo area fields. Our results suggest that seedling freezing tolerance could be easily improved in new varieties and enhancing freezing tolerance by breeding may be a worthy goal to ensure long term persistence, although the perennial Lewis flax already has excellent survival characteristics, on average. This work was submitted for publication to Annals of Botany-Plants and is currently being revised on the advice of peer scientists. The final paper is expected to be published as an open access article in Annals of Botany-Plants in early 2024.

Figure 14. (top) Results of freezing tolerance tests of 45 Lewis flax collections representing the full range in latitude, longitude, and elevation of collection site (L to R) soil and air temperature of 15 F, 10 F, and 5 F. (bottom) Regrowth vigor of the Maple Grove variety over three weeks after freezing at equal temperature increments from 27 degrees F (yellow line) to 5 degrees F (purple line). The flat line at 5 F indicates no regrowth because of plant death. Most accessions possessed better freeze tolerance than Maple Grove.

During the height of the pandemic, we learned of plant collection and characterization work that was unpublished by Tom Jones, USDA-ARS, Logan, UT, and Stan Kitchen, USDA-NRCS, Ephraim, UT. They have collected wild perennial flax from the intermountain west and evaluated it for traits of interest for disturbance restoration plantings. These data were used to release the 'Maple Grove' variety, but were otherwise unpublished. Andre Gossweiler, a graduate student on this project, and Peter Innes, a graduate student of a collaborator at University of Colorado-Boulder, analyzed the data and wrote a peer-reviewed publication summarizing their findings, which also have importance for agriculture. Andre also contributed oil content and composition data of the samples from the Utah common garden experiment. The results provided a lot of useful information on trait correlations that will benefit increasing seed size and oil content by breeding, as well as altering plant architecture to be favorable for cultivation. This manuscript has been published in Annals of Botany-Plants, a scientific journal with a high impact factor rating of 3.276.

Compared to when we began breeding work on Lewis flax, we have a fuller appreciation of the domestication traits that need to be included in Lewis flax varieties for seed harvest. As a final, but important, part of our breeding activities, we needed to develop a reference genome sequence of Lewis flax in order to permit genetics studies and marker-assisted breeding. This will allow for more rapid breeding of domestication traits (reduced boll shattering, determinate habit, improved autonomous selfing, etc.) by defining DNA-based markers for the traits that can be scored at any time in the life of a plant. This reference sequence publication is now available as a pre-print, and the data are available in public archives.

Outreach: While Covid has certainly limited the opportunities for outreach, our partnership with Noreen Thomas has been helpful for informing the public about our work, through her great network and her abilities as an educator. Noreen had many visitors to her farm during the project, even during Covid, and has facilitated tours of our perennial flax nursery with researchers/students from the University of Minnesota, Iowa State University, and elsewhere; other farmers with a summer farm tour event; interested members of the public; and even a virtual tour with New York City chef Dan Barber, which is turning into a new collaboration on the food aspects of perennial flax in collaboration with Stone Barns in upstate New York. We spoke to a large group (~200 individuals) at North Dakota State University’s Dale E. Herman Research Arboretum annual field day located near Absaraka, ND. We were invited to present our yearly results at the Northern Plains Sustainable Ag Society conference in Fargo, each year. Some of our preliminary results were shared at the virtual American Society of Agronomy annual meeting in November 2020 and live American Society of Agronomy meetings in Salt Lake City, UT, in 2021 and Baltimore, MD, in 2022. In addition, although short-handed because of Covid and other health crises in our group, we have been able to produce two YouTube videos on perennial flax, one within our group and one with the assistance of NDSU extension. We plan to continue tours at perennial flax field sites in the future.

Our presentations at field days and other public events and conferences focused on the background, scope, and progress of our work with perennial flax. Audience members included neighbors, farmers (organic and conventional), students, and professors. Engagement from audiences was very high. Audience questions focused primarily on the economics of perennial flax production with a particular interest in yield potential and markets. The overwhelming response we receive indicates considerable interest in perennial Lewis flax for cultivation. Producers are encouraged by our research in organic weed control and the agronomic implications of perennial flax cultivation. End users are interested in the fatty acid composition of the seed as an important source of omega-3-rich vegetable oil and seed products.