How to Use Season Extension Effectively for Winter Market Sales: Investigating Planting Date, Types of Cover, and Fertility

Despite the setbacks, problems, and difficulties, there are some very useful points discovered and important information to share with farmers. Due to the heat, getting transplants and seedlings established was frustrating. The timing was totally set off when we tried to make 4 successive planting dates. The first section to review is how the varieties performed. Table 1 shows planting dates for transplants and direct seeding dates for the rest. Harvest dates are listed as well and this represents the date at which the first harvest begins. Some of the varieties would be harvested over a course of days and weeks.

The lettuce varieties trialed were Jericho, Rouge d’Hiver (both romaine-types), Red Sails (looseleaf) and Winter Density (Bibb). The first planting of these transplants struggled to get going under hot conditions. Despite losing nearly half at the outset, consistent watering helped settle the survivors in though the eventual head sizes were small. As the fall temperatures dropped, heads were able to be kept in the tunnel and picked as needed.

Farm 3 was in an area that had cooler temperatures and heavier soils that stayed moister. Establishment was easier, head size was better, and harvest dates were more in line with what was expected. All the varieties were trialed for at least 3 planting dates. The first planting date for all the varieties trialed were harvested in the shortest amount of time while each succeeding harvest was longer. Jericho: from transplant to harvest – planting date 1 = 35 days; planting date 4 = 79. Red Sails transplant to harvest – planting date 1 = 35; planting date 4 60. Rouge d’Hiver transplant to harvest – planting date 1 = 42; planting date 3 = 60. Winter Density transplant to harvest – planting date 1 = 50; planting date 3 69.

This pattern holds true pretty much for all of the different greens varieties trialed. Again, decreasing light is the limiting factor along with cooler soil temperatures. As the temperatures get colder, adding the first layer of row cover, Agribon 19 eliminates almost 20% of the light through the material. Couple this with decreasing daylight really reduces growth while increasing days to harvest dramatically.

Farm 2 had only tried one planting of Jericho and like Farm 1, did not have good results.

The second planting at Farm 1 looked to be the best of the 4 attempts in 2009. Growth was slower than in the spring season, probably having to do with diminishing duration of sun light. The harvest had the most heads of large size of all the planting dates. The heads were held in the cool conditions until needed for the opening of the winter market in November.

The third and fourth planting of Jericho took several weeks longer to reach even average size. The fourth harvest required that two heads be sold together to make up for the smaller size. Some of the heads were left in the tunnel to overwinter.

In March 2010, Jericho was harvested out of the tunnels. It wasn’t full sized but large enough to sell mostly as single heads. There was noticeable growth but it was later than estimated by two weeks. The quality was good and the decision to harvest was made based on worry of too much warmth and the increasing sunlight would cause bolting and bitter flavor.

For the 2010 fall season, late summer proved to be even hotter than the previous year. This set back seeding for transplants by several weeks. Farm 1 and 2 both had mid-November harvests off their first planting which actually did pretty nice. Heads were green and tall.
Farm 1 tried three more successive plantings with the second being harvested close to maturity in December while the third and fourth plantings were too small and attempts were made to carry them over the winter.

Red Sails did produce nice heads with rich color. The cooler temperatures latter in year brings out the reds darker than during warmer weather. The first planting was paler than the later plantings. This looseleaf variety by itself looked good but putting it up to the romaines like Jericho and Rouge d’Hiver, Red Sails didn’t appear to be as full and appealing.

Rouge d’Hiver had a sharp red coloration on the outer leaves with darker green towards the center. It took longer to grow but the heads were as large as or larger than Jericho for the first and second plantings. These also held up well overwinter into March. The 4th planting did increase in size in March but the harvest date was late into the month and the flavor was starting to get bitter. The mid-March harvest of the third planting date tasted far better.
Winter Density only had one planting date for Farm 1 in 2009 and 2010. This variety really does not like hot temperatures and had poor germination and slow growth. For Farm 3, the later plantings faired better.

Swiss Chard – Bright Lights takes a while to get going from seed in the heat. Production was slow and harvest of sizeable leaves only amounted so very small bunches. The lesson learned here is to get plants established in the spring and keep them growing throughout the summer and then cover them for late season use as mature plants from the beginning. In this way, time isn’t wasted waiting for the plants to gain size which is very slow as light diminishes. The larger plants also seem to survive better over winter than young plants do as well.

For kale, both Winterbor and Redbor, the same is true as with Swiss chard. Get the plants established in the spring. It takes too long to get any size to the plants to make it worth while harvesting for the amount of space it takes up in the tunnel. The plan should be to have kale, chard, cutting celery, and parsley all started in the spring in their own separate row together. When the fall comes, build a tunnel over the established plants and protect them for the late season. Then harvesting would be coming from full sized plants and they also overwinter under protection for late winter and early spring harvests.
Both varieties grew once the weather cooled down. The colors were dark. The textures were crisp and the flavor was sweeter with the lowering temperatures. The size of the plants were small, however, making them not very productive overall.

Spinach: the varieties were Space, Tyee, Regiment, and Renegade. Spinach geminates poorly in warm weather and not at all in hot weather. Getting stand establishment was slow. Daily irrigation was needed to get any germination after several attempts at seeding. Many growers now transplant spinach to avoid direct seeding. So looking at the initial results doesn’t show anything great for the first two planting dates in 2009. The later planting dates did better with full sized plants. Regiment and Renegade produced the largest bunches.
Mustards: the varieties were Mizuna, Tat soi, Yukima Savoy, and Pak choi – Joi Choi. Again, seed germination was poor initially. The stress on mustards results in bolting Tat soi appears to be the most sensitive to this. The second, third, and fourth seeding dates had better germination and plant growth for all the varieties. Mizuna and Yukima Savoy did well with full growth. Yukima Savoy also survived overwinter. Joi Choi did well with the second and third seeding dates producing full sized heads. The fourth planting dates had smaller heads but were sold as “baby choi”.

Arugula and Cilantro – Santo also failed miserably due to the heat early on. Later seedings of arugula germinated and grew but not as well as with spring plantings for Farm 1 while it did better at Farm 3. The plants at Farm 3 grew to full size and had several cuttings harvested.

Santo cilantro eventually germinated once the temperatures cooled down but germination was sporadic. This might do better planted in earlier and in the shade of another crop that could be harvested out of the way in time for the cilantro to take off in mid-September. Something interesting to note was that the cilantro did germinate in March after overwintering and produced a very strong early season crop.

Broccoli rapi – Sessantina: This variety did well as long as it was kept watered. Stress causes bolting. The plants can be harvested for greens or shoots (small broccoli florets). The plants sized up in reasonable time. The flavor improved with cooler weather. Getting customers to try this vegetable green and use it in cooking is a challenge but with the right market, rapi can be a good choice for season extension production.

Tunnel Temperatures and Covers

Table 2 shows the temperature data for three sites in 2009/10 and 2010/11. The data is consolidated from piecing together what we could collect trying to overcome problems with two batches of max/min thermometers purchased that were defected. So instead of showing three sets of temperature data the best display of the data is to put it together to give what the fall and winter looked like over two seasons. Some weather data was also collected from the NEWA weather site for the area to fill in a few of the dates for outdoor temperatures.

The plan was to cover the tunnels using the thinner row cover fabric, Agribon 19. This is rated to reduce sunlight by 19%. We would use this while the temperatures were in 40’s during the day and 30’s at night. By mid-November, the Agribon 19 would be replaced with Agribon 30. It would offer more protection against the cold though it would reduce the light by 30%. By early December, there wouldn’t be enough sunlight to really allow for plant growth so at this point protection against freezing would be the most important. Covering over the row cover with the greenhouse sheeting would then be used. This would trap more heat under the tunnel, cut down wind-drying issues, and offer a thicker blanket to hold the plants over for harvest then overwintering for the future harvest in late February and March.

In 2009, the night temperatures were dropping low enough to warrant covering with Agribon 19. With warm sunny days, the covers were removed. The row cover was held down by sand bags. After a week or two of covering and uncovering, the Agribon was left on. The heat and humidity increased inside the tunnels and the growth improved. Soil temperatures were high to start and slowly dropped over the weeks through mid-December (8/15 – 74Fo; 12/29 – 50Fo.).

On Dec. 5, the Agribon 19 was replaced with the heavier weight Agribon 30 and the clear greenhouse poly sheeting. This offered extra protection for the plants and moderates the lowering soil temperature. As the weather really turned cold, the soil temperature dropped quickly into the low 30’s.

One of the main problems encountered occurred on 1/11. The edges of the row cover became solidly frozen to the ground. This makes entering the tunnels impossible without ripping the poly sheeting or tearing the fabric. Checking the soil temperature inside the tunnel wasn’t possible during these weeks. Harvesting greens also was halted.

The second major issue that occurred repeatedly was high winds pulling off the row cover, especially on farm 1 and 2 which ran their tunnels right through the winter. The sand bags seemed to be heavy but as the winds caused the row cover to ripple, the effect was to pull the edges of the cover out from under the sand bags little by little. With more slack in the cover the wind speed forces the row cover to pull itself totally free and flaps around. If it snows, then the cover gets buried making it harder to pull back on. The cost of labor for recovering the tunnels adds up when this happens. The quality of the plants decreases from freezing or wind injury. All that effort exerted to just lose plants to the wind. Eventually, to fix this problem, sand bags on either side of the tunnel were tied together with the cord going over the tunnel. With sand bags at the base of every hoop, the cord goes over the tunnel to the corresponding sand bag. The cord is just tight enough to prevent the row cover from rippling and flapping in the wind.

What we discovered was that switching from the lighter row cover to the heavier and adding the greenhouse sheeting provided important protection for the plants. One small tunnel was set up for on one farm for other greens and was covered with only Agribon 19 for the winter. The inside temperature dropped frequently into the low 20’s or lower and the greens froze solid. Many of the plants were damaged or died. For the tunnels with the heavier row cover and greenhouse plastic allowed the plants to survive in good shape (marketable shape). When the temperature and light levels rose in late February and early March, growth resumed. By mid to late March, marketable sized plants were ready again.

Nitrate Testing

The last segment we examined was nutrient availability and accumulating nitrates in the greens. We set up a tunnel with four sections having a control of no added fertilizer and the next three with increasing amounts of fertilizer. To get quick ballpark results, a refractometer was used. Refractometers give a reading of sugar and mineral levels in plant tissue. Two farms participated in this segment of the trial. Readings were taken in November, December, and March for those vegetables that were present at that time for those farms. This data is found in Table 3.

Comparing the readings with a chart of tested vegetable Brix values (chart available with refractometer and also available from Peaceful Valley Farm Supply www.groworganic.com) , a determination of low, average, or high percentage of soluble sugars and minerals are present. Since there is a direct relationship between the sugar levels and N accumulated in the plant tissue, this will provide an estimate of nitrate in the leaves.

Collecting leaves and crushing them to get sap to place into the meter gives a fast reading. The meter is first calibrated using distilled water to get a reading of 0. Several drops of plant sap is added to the meter plate and closed. Holding the meter up to the light, refracting it gives a manual reading of numbers ranging from 1-26.

From each farm, four samples of leaves were taken from the most mature variety in mid- November as a base line. Another round of samples were taken in mid-December. It was felt that by this time, the plants would have taken up the most nutrients they were going to since the soil temperatures had dropped. Light levels were about at their lowest points. A follow-up round of samples was taken in early March to see if there were any changes.
The results of the Brix readings for November ranged low to moderate. Mizuna ranged from 5-7 in November averaging around 6. In December the range was 5-9 averaging around 7. In March the average was still 7 though the range was 6-8. The chart lists 6 as average and above 10 as high.

With spinach, Regiment was tested. In November, the range was 5-7 averaging close to 6.5. In December the range was 6-8 averaging about 7. For March the range increased to 7-9 averaging 7.5. There was no chart listing for spinach.

Winterbor kale was tested and in November ranging 7-10 averaging about 8.3. In December the range widened to 7-11 with an average around 9. Then in March the average stayed around again with a range of 7-10. The chart lists kale with an average of 10.
Lettuce had a range of 4-8 in November with an average just under 6. In December the range was 6-9 averaging around 7.7. In March the range was 5-8 with a lower average about 7. The chart list lettuce with an average of 6 and above 10 as high.

Arugula ranged from 5-8 across the three testing periods averaging around 6. The average of 6 is found on the chart.

Swiss chard ranged 6-8 and the chart lists the average at 8.
Leaf samples were also collected in December for both farms (only one set from one farm in January) and sent off for tissue analysis. The tests will provide the levels of nitrate in ppm found in the samples. Having numbers in the 4200 – 4800 range suggests that nitrates are in the acceptable levels. Anything below that is low and above is high.
The results returned did not vary too widely for the most part. Mizuna had all but two samples at or below average. One of the two outlying samples was just above 5200 while the other was nearly 8000. The January samples were at or below average.

For spinach the samples were at or below average.
For kale, the samples were generally average or below with a couple of samples slightly above average. This was true for the January samples as well.

Lettuce had the lowest readings of all the greens sampled. Most of the samples were well below average with 8 readings below 3500. One reading was 916.

Chard and arugula were also mostly below average with arugula more on the low side of the scale.

Pulling all this information together basically confirms that despite adding more N for fertility, nitrates are not accumulating to above average levels in the leaves. This is good. We don’t want a toxicity issue developing as more farmers turn to season extension production. Since there were several samples did have higher than readings, the issue of nitrates should be made of aware to growers. Perhaps the organic fertilizer used did not fully break down and was made available to the plants due to reduced soil biological activity. If high rates of fertilizers are used earlier than we had applied them or if a more soluble form was added, then nitrates might become an issue.

Using a refractometer might be a useful tool if baseline readings are first made. Then if the is sharp rise in Brix readings, then a follow up tissue sample analysis should be done to err on the safe side. Many stage agricultural colleges provide tissue analysis services as do several private labs that do soil sample analysis.

• Preparing tunnel to be covered
• greens in tunnel
• Some of the varieties of greens in trial
• Testing For Nitrates
• Temperature data