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Does extra greenhouse light make young grapevines more fruitful?
North Carolina State University researchers found that supplemental LED light in the greenhouse made young Traminette grapevines more fruitful once they were planted in a vineyard, although yields did not differ between light treatments and Concord vines developed bud necrosis in every treatment.
Plant a new bareroot grapevine and the first harvest can be two to five years away, depending on the region and climate. A team in the Department of Horticultural Science at North Carolina State University wanted to know whether a season under extra light in a greenhouse could set young vines up to crop in the same year they go into the ground (Freedman et al., 2026). They tried it on two cultivars, ‘Traminette’ (a French-American hybrid) and ‘Concord’ (Vitis labrusca), then followed the vines through two seasons in a commercial vineyard.
What did the team do?
The work ran in two phases. First, one-year-old bareroot vines were potted into grow bags at the NC State University Horticultural Field Laboratory in Raleigh, rooted and trained to a single shoot. After 32 days, 48 vines (24 Concord and 24 Traminette) were chosen from the 60 they started with, picking vines as close as possible to trellis height, and moved to a glass greenhouse.
There, each trellis held two Concord and two Traminette vines under two LED top lights, and the vines got one of three treatments for 101 days: no supplemental light (NSL), medium supplemental light (MSL, using 300 micromoles per square metre per second of supplemental lighting) or high supplemental light (HSL, using 600). Over the whole treatment period, those added up to cumulative daily light integrals of 887 ± 113, 2895 ± 173 and 4928 ± 302 moles per square metre.
The team measured growth, photosynthesis and dry mass, then dissected buds from half the vines under a stereomicroscope to count inflorescence primordia, the early bud structures that later develop into clusters. The paper calls this count potential bud fruitfulness. They used a method they call bud isolation, removing the whole compound bud first so the primary, secondary and tertiary buds could each be examined. The rest of the vines spent the winter dormant, then were planted at FireClay Cellars, a commercial vineyard in Siler City, North Carolina, in May 2022. There the team counted the clusters that actually appeared on Traminette (observed bud fruitfulness) and harvested the vines in 2022 and 2023.
What did they find?
- More light, bigger vines. Both cultivars had significantly higher dry mass under HSL than under NSL and MSL. Traminette dry mass was 46% and 92% greater than NSL under MSL and HSL respectively, and Concord’s was 39% and 79% greater. Traminette root dry mass was significantly higher by 104% and 127% under MSL and HSL than NSL, while light had no significant effect on Concord root dry mass.
- Leaves worked harder. At 62 days, net photosynthetic rate was 67% and 69% greater for Concord, and 91% and 115% greater for Traminette, under MSL and HSL respectively. There was no difference between MSL and HSL.
- The gains were in the secondary and tertiary buds. Dissections found no significant difference in the primary buds. Secondary buds had significantly higher potential fruitfulness under HSL than MSL, and tertiary buds were significantly higher under HSL than both NSL and MSL.
- More clusters in the vineyard. After planting out, total observed bud fruitfulness in Traminette was 2.8 ± 1.07 (NSL), 3.5 ± 1.56 (MSL) and 3.9 ± 1.30 (HSL) per node, significantly higher for MSL and HSL than NSL. The difference came from secondary and tertiary buds, and MSL and HSL did not differ significantly from each other.
- But no extra fruit. Cluster weight, berry weight and yield per vine showed no significant differences between light treatments in either year. Every Traminette vine yielded a crop in the year of planting and again the following season.
- Concord struggled. Concord showed extensive bud necrosis in all treatments, so its buds were not evaluated for potential fruitfulness, and in the vineyard Concord vines showed almost no shoot growth.
In short, the extra light went with more clusters on Traminette in the vineyard, but not with more kilograms at harvest.
What could it mean for growers?
The authors conclude that supplemental light in a controlled environment greenhouse improved potential and observed bud fruitfulness in Traminette, and that conditioned Traminette vines could be cropped in the season they were planted and the season after. Because the extra fruitfulness did not translate into yield, they say pre-conditioned grapevines will need new management methods in the field, due to higher crop loads and the potential retention of secondary and tertiary clusters for yield. They also suggest that this kind of greenhouse conditioning has to be adjusted to specific cultivars, given how differently Concord responded. In their view, controlled environment applications in viticulture could potentially increase planting flexibility and reduce operational cost.
It is not a finished method yet. The authors call for further work on cultivar-specific greenhouse treatments, field management, the system’s economics and running it at scale.
What were the limitations?
- Vines were selected for height close to the trellis, and the authors note that picking the larger vines may have selected towards more vigorous vines.
- The cause of the Concord bud necrosis is unknown. The authors suggest it might relate to water and nutrient management in the greenhouse, and that fertilisation may have been too high for own-rooted Concord.
- Inflorescence primordia can be very small and hard to identify under a stereomicroscope, especially in one-year-old vines, so some may have been present at dissection but too small to count.
- In the vineyard, the vines were managed under standard North Carolina methods for mature grapevines, in a vineyard with high disease pressure. In the year of planting there was a summer drought period during critical stages such as bloom. The authors say management and environmental factors on the farm could explain why yields were similar across treatments.
- One NSL vine suffered severe damage from Pierce’s Disease and was excluded from the data.
- The two cultivars were not compared with each other, because their genetics and plant material (own-rooted versus grafted) differed.
Trial details
- Where: greenhouse phase at the North Carolina State University Horticultural Field Laboratory, Raleigh, North Carolina, United States: an 85 m2 plastic greenhouse for rooting and establishment, then a 156 m2 glass greenhouse for the light treatments. Field phase at FireClay Cellars, a 2.8-hectare commercial vineyard in the Haw River American Viticulture Area, Siler City, North Carolina (Georgeville silty clay loam, 2% south-facing slope, rows 2.74 m apart and vines 2.13 m apart, vertically shoot positioned trellis with a wire at 0.91 m).
- Crop and stage: one-year-old bareroot dormant grapevines of ‘Concord’ (Vitis labrusca, own-rooted) and ‘Traminette’ (Joannes Seyve 23.416 x ‘Gewurztraminer’, grafted on 101-14 rootstock), purchased from Double A Vineyards (Fredonia, NY).
- How long: rooting and establishment for 32 days in May 2021; light treatments for 101 days, from 28 June to 02 October 2021; bud dissections between October 2021 and January 2022; dormancy for 201 days; vineyard planting on 03 May 2022; harvests on 19 August 2022 and 22 August 2023.
- System and substrate: grow bags (37 L in the abstract, 10 gallon in the methods) of Jolly Gardener PRO-LINE C/P Growing Mix, on custom PVC trellises about 1.5 m long x 0.97 m wide x 0.91 m high. Drip irrigation, with each trellis’s valve opened for 25 minutes when substrate volumetric water content reached or fell below 0.345 ± 0.014 m3 m-3 (80% of container capacity).
- Lighting: two LED top lights per trellis (Arize Element L1000, Current Lighting). Supplemental PPFD of 7.7 ± 2.26 (NSL), 300.4 ± 4.27 (MSL) and 603.5 ± 5.22 (HSL) μmol m-2 s-1; DLI of 8.8 ± 1.8, 28.7 ± 2.3 and 48.9 ± 3.8 mol m-2 d-1; cumulative DLI of 887 ± 113, 2895 ± 173 and 4928 ± 302 mol m-2. Photoperiod given as 17 h day-1 in the text (supplemental light from 0500 to 2200 EST) and 18 hours in Table 1. Spectrum in Table 1: 17.7 ± 0.5% blue, 0.2 ± 0% green and 82.1 ± 0.6% red with a red to far red ratio of 136.7 ± 17.7 for the treatments, against 25.3% blue, 36.0% green, 38.7% red and a ratio of 1.2 for sunlight in the greenhouse. During establishment, DLI at canopy level was 8.49 ± 2.69 mol m-2 d-1.
- Climate: establishment phase 25 °C ± 5.17 and relative humidity 77.99 ± 18.04%; light treatment greenhouse 25.6 ± 0.4 °C and relative humidity 80.35 ± 9.78% (Table 1, where the humidity value sits under the no supplemental light and medium supplemental light columns); during dormancy outdoors on a nursery pad, average 24-hour temperature 9.1 ± 7.7 °C.
- Nutrition: fertigation through injectors (solutions A and B) with four drip emitters per container; nutrient solution pH 5.98 and EC 1.62 dS m-1, with 140 ppm total nitrogen (15 ppm NH4, 125 ppm NO3). In the vineyard, a low dose of 50 ppm 20-10-20 water-soluble fertiliser applied by hand twice in May.
- Design: randomised complete block design with three light treatments, each replicated four times (12 experimental units per cultivar); each trellis of four vines held one Concord and one Traminette experimental unit (n=48). Two replicates (12 Concord and 12 Traminette vines) were evaluated destructively at the end of the greenhouse phase; the remaining 24 vines went through dormancy and to the vineyard. Net photosynthetic rate measured at 62 and 97 days after transplanting. Analysis in JMP Pro 17 using two-way ANOVA (α = 0.05) with Tukey’s HSD, or Kruskal-Wallis with post-hoc Wilcoxon Each Pair tests for non-parametric data.
- Results: Traminette total dry mass 214.8 ± 17.3 g (NSL), 312.7 ± 11.0 g (MSL) and 411.9 ± 17.4 g (HSL); Concord 124.3 ± 17.5 g, 173.2 ± 11.1 g and 222.7 ± 17.6 g (means ± SE, n=12). Traminette potential bud fruitfulness, total per node, 3 ± 1.6 (NSL), 2.5 ± 1.7 (MSL) and 3.4 ± 1.6 (HSL); tertiary buds 0.5 ± 0.7, 0.3 ± 0.6 and 1 ± 0.8 (means ± SD, n=12). Shoots per node in the vineyard averaged 1.6, 1.9 and 2.1. Traminette yield per vine 1.15 ± 0.12, 1.13 ± 0.07 and 1.11 ± 0.12 kg in 2022, and 0.76 ± 0.29, 0.73 ± 0.18 and 0.69 ± 0.29 kg in 2023 (n=8 in the table notes), with no significant differences. No significant difference in node number, internode length or leaf area index between treatments for either cultivar.
- Funding and conflicts of interest: funded by the North Carolina Department of Agriculture Specialty Crop Block Grant Program (Grant No. 21-098-4007) and the NCSU CALS Innovation Grant Program. The acknowledgements state that GE Current Lighting provided material support of LED lights. The authors declare no commercial or financial relationships that could be construed as a potential conflict of interest.
- Publication: Frontiers in Horticulture, volume 5, article 1953764 (section Controlled Environment Horticulture), published 3 November 2026, https://doi.org/10.3389/fhort.2026.1953764. Peer reviewed. Licence: CC BY 4.0.
- Not reported in the paper: separate day and night temperatures; VPD; greenhouse CO2 concentration (only the 400 ppm setting inside the photosynthesis chamber is given); pH and EC of the vineyard fertiliser.
Links
- The study: Impact of supplemental light on bud fruitfulness, physiology and vine yield of containerized ‘Traminette’ (French-American hybrid) and ‘Concord’ (Vitis labrusca)
- The institution: Viticulture, NC State Extension (North Carolina State University)
References
- Freedman, K., Heagy, K., Collado, C., Hernández, R., & Hoffmann, M. (2026). Impact of supplemental light on bud fruitfulness, physiology and vine yield of containerized ‘Traminette’ (French-American hybrid) and ‘Concord’ (Vitis labrusca). Frontiers in Horticulture, 5, 1953764. https://doi.org/10.3389/fhort.2026.1953764