One of the most common difficulties that winter spinach growers in the Northeast have is achieving good germination and stand in high tunnels. Germination is often patchy, and pre- and post-emergence damping off, caused by several fungal and fungal-like pathogens, is hard to avoid. These diseases are caused by fungi in the genera Rhizoctonia and Fusarium and fungal-like organisms in the genus Pythium, which are weak pathogens that only attack young, weakened, or slow-growing plants. They build up in the soil when crops are grown continuously, with no fallow period for the soil and its microbial community to recover. Incorporating fresh organic matter, especially in the form of cover crop residue is one well-documented way to support growth of healthy microbes in soil and reduce incidence of damping off.
In the fall of 2021, we investigated a few different strategies to improve spinach stands in winter tunnels by reducing damping off and/or improving spinach germination. We tested three factors:
- Incorporating cover crop residues into the soil pre-plant. Soil microbes, including both beneficial and pathogenic fungi and fungal-like organisms, feed on organic matter in the soil. Soil microbes compete for resources and space, and pathogenic soil fungi are relatively poor competitors. By adding fresh organic matter to the soil, we hoped to provide more resources for the whole soil microbial community and give a boost to beneficial soil fungi that could out-compete pathogenic fungi. We chose buckwheat as our cover crop because it could potentially be grown quickly between summer and fall high tunnel crops and has been shown to reduce damping off when incorporated 3-weeks before planting.
- Priming the seed to speed up germination. Priming is a process of soaking seed before planting in order to jumpstart the germination process, resulting in faster germination after seeding. Damping off pathogens only infect young or weakened seedlings, so we hypothesized that the faster seeds germinate in the soil, the less time damping off pathogens have to infect the germinating seeds, resulting in lower incidence of damping off and better stands. Solutions used for priming seed vary from just water, to water + hydrogen peroxide, to solutions with chemicals added to control the osmotic potential in order to limit how much liquid the seeds take up. We did not find a common, scientifically-verified procedure for priming spinach seed, so we spoke to an expert on seed science, Dr. Alan Taylor of Cornell University, and developed the method described below. We wanted to test a protocol that would be simple for farmers to replicate on farms using materials they could easily source.
- Spinach varieties vary widely in their germination speed and uniformity, and the effects of seed priming are known to vary by variety, so we also included two varieties in our trial. For this trial, we compared the effects of cover cropping and seed priming on Kolibri, which is commonly grown in Northeast tunnels, and Crosstrek, a newer variety that has performed well in our recent variety trials.
Trial Setup
Cover Crop
To evaluate the effects of incorporating cover crops on spinach germination, the high tunnel was split in half. Over the summer prior to planting the spinach trial, one half was planted into cucumbers for an unrelated trial, and one half was planted into a cover crop in preparation for the spinach trial.
For the cover crop treatment, buckwheat was broadcast-seeded into half of the tunnel on July 23 at a rate of 1.39 lbs/1000 ft2 (60.5 lbs/A). Due to poor germination, the buckwheat was reseeded at a rate of 2.08 lbs/ft2 (90.75 lbs/A) on July 30. The seeding on July 30 was raked by hand to incorporate and then irrigated for 4 hours. The buckwheat was irrigated regularly throughout July and August, then mowed on September 9 and rototilled to incorporate on September 10, three weeks before planting spinach.
The non-cover cropped half of the tunnel was planted with cucumbers grown on white plastic beds over the summer of 2021. This side of the tunnel was amended with 50 lbs/A of nitrogen in the form of 5-4-8 chicken manure prior to the cucumbers being planted. The cucumbers and the plastic mulch were removed in late-September.
Priming
The spinach seed was primed overnight the day before seeding. The seeds were soaked in a 0.3% hydrogen peroxide solution for 3 hours, then were drained and put into a container where they remained damp for 16 hours at 65°F, allowing the seeds to slowly imbibe water overnight. The next morning, the container lids were removed and the seeds were spread out to dry at room temperature before planting.
Spinach was seeded into the tunnel on September 31, at a rate of 3 million seeds/A (70 seeds/ft2). Plots were 2 ft x 6 ft, with 1 ft between plots in-bed. Within each cover crop treatment, plots were arranged in a randomized complete block design with each factor replicated 4 times within the cover cropped side and the non-cover cropped side. The tunnel was overhead irrigated as needed throughout the winter. From September 31 to November 2, the tunnel sides, end wall doors, and end wall vents were open. On November 2, the end wall doors were closed and the sides were programmed to close at 40°F and open at 50°F; end wall vents remained open. There were no exhaust or circulating fans running throughout the course of the trial.
Over the course of the trial, air temperature ranged from 28.9 to 77.7⁰F, and soil temperature 3 inches below the soil surface ranged from 40.0 to 72.1⁰F. Both air and soil temperatures were highest at the beginning of the trial, in mid-October, and both were coldest in early to mid-November, just before the end of the trial.
Germination was rated by counting the number of plants in 2 row feet 1 week after seeding. Post-emergence damping off was rated on October 12, 15, and 19 by counting the number of wilting or dead plants in 2 row feet. Plot vigor, rated as a percentage, was also rated on all of those dates and twice a week from October 22 through November 16. All plots were harvested on November 18 and yield data was collected.
Results
Statistical analysis was conducted using a general linear mixed model including all main effects and interactions. While priming was significant (p = 0.0477), using the priming protocol we developed had no positive effect on germination speed—plots with primed seed actually had lower germination rates than plots with unprimed seed. Thus, we used the unprimed data for the rest of the analysis, leaving cover crop and variety as our two main effects. The incorporation of cover crop residue had the largest effect (p = 0.0001) on germination, vigor, and yield, though variety was also significant (p = 0.0392), with Crosstrek outperforming Kolibri in all measures. All treatments reached their maximum vigor (all above 80%) and yields (all 0.58 lbs/ft2) in the cover crop plots (see Figures 1 and 2). The interaction of variety and covercrop was not significant (p=0.4550), meaning the effect of cover cropping was the same across the two varieties—cover 
crop residues led to increased germination, vigor, and yield no matter the variety.
Discussion
While we had hypothesized that turning in fresh cover crop residues would stimulate activity of beneficial soil microbes and reduce damping off, we were surprised by the scale of the difference we saw in germination and growth on the two sides of the tunnel by mid-October. So, we tried to determine what else might have contributed to these differences in growth.
The bare ground half of the tunnel had been planted to cucumbers the previous summer, while the cover-cropped half was in buckwheat all summer. No fertilizer was added to either side before the spinach trial was planted. On October 29, pre-sidedress nitrate tests showed that the soil nitrate in the cover-cropped beds was double that of the bare ground beds—32 compared to 15 ppm. Since we did not control for soil nitrate content between the cover-cropped side of the tunnel and the bare ground side, the effects we saw from cover crop incorporation may have simply been the effect of higher soil nitrate. We hope to conduct further experiments to tease apart these two variables in the future.
Conclusions
The significant trends that we saw from this study were:
- Incorporation of a buckwheat cover crop three weeks before seeding (and/or higher available nitrate at seeding) resulted in maximized germination, vigor, and yield of winter high tunnel spinach. This difference could also be attributed to nitrogen fertility in the plots, further study is needed to tease this apart.
- The variety Crosstrek consistently out-performed Kolibri.
- Priming seed using the protocol we developed reduced germination and yield of both varieties.
Taking a step back, it’s clear that variety selection can have a big impact on germination, and therefore on yields. It can be difficult to access the newest spinach varieties through New England distributors, as most spinach is marketed to large-scale, West Coast producers. Distributors are often able to special order unlisted varieties, especially if the order meets a minimum size, so work with neighboring farms to meet minimum orders! One of the most important recommendations for managing spinach downy mildew, an important disease of winter spinach, is to grow several varieties with varying gaps in DM resistance. If you follow 
this recommendation and grow several varieties, pay attention to differences in germination, and adjust your variety choices based on your observations.
The effects of cover crop incorporation in this trial were clear – plots with buckwheat incorporated significantly outperformed those with no buckwheat – but the mechanism behind those effects are unknown. More research is needed to investigate whether the benefits we saw from buckwheat incorporation were from the addition of fresh organic matter to the soil, or whether they were from the higher residual nitrate, or both.
Lastly, while priming as we did it clearly had a negative effect on germination, there are many other priming procedures—different priming solutions and treatment times—that might have different effects. More research in this area could also be useful.
This material is based upon work supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, through the Northeast Sustainable Agriculture Research and Education program under subaward number LNE20-402.
