Spinach (Spinacia oleracea L.) can be harvested as baby spinach (soupspoon sized leaves) with repeated cuttings from the same planting. Spinach planted in the autumn can be harvested with repeated cuttings through the winter and into the spring. Autumn planting date is critical to winter harvests.  Through the short cold days of winter spinach continues to grow, but at a much reduced rate.  This growth reduction takes effect around 16 November at the 39 °N latitude (Coleman, 2001).  Autumn crops must grow vegetatively before this time to carry the crop through the winter.  Coleman (2001) presents his high tunnel autumn planting dates as a frame of reference for other regions.  In Maine, he plants in an unheated high tunnel through September to harvest through the winter.

          Planting date affects yield and growing days before harvest based on the accumulation of heat units (growing degree days) by the crop and on light intensity.  To begin to assess the effects of autumn planting date on spinach harvest in Kansas high tunnels, we planted spinach on six dates in the autumn of 2005 at the Kansas State University Horticulture Research and Extension Center, Olathe [latitude 38°53'N, elevation 1056 ft (322 m)]. Growers may be able to use information from this study when choosing autumn planting dates for spinach.  Planting date choice would differ based on need for autumn, midwinter or spring harvests.
 

Fall spinach

Author Sharon Knewtson in her research plots

         
          The experiment was conducted with two cultivars – Avenger (from Seminis, Inc., St. Louis, Mo.) and PVO172 (from Santa Clara Seeds, Greenfield, Calif.).  Each cultivar was replicated with plantings in three organically managed high tunnels and three conventionally managed high tunnels.  The spinach was harvested when the leaves reached soupspoon size.  Yield was measured as fresh mass.  Harvest dates differed between planting dates and cultivars.  Statistical significance is reported with possible 5 percent error (p = 0.05).

          Seed beds were 30 ft long and 2 ft wide, divided into six plots of 5-ft length.  Fertilizer was applied at a rate to provide the equivalent of 200 lb N per acre.  Organic fertilizer Hu-more compost (1-1-1) from Humalfa, LLC. (Shattuck, Okla.), and a conventional pelletized fertilizer (16-8-8), were used.  Planting dates were 6, 11, and 25 October, and 3, 8, and 17 November.  Planting dates were randomly assigned to plots within a bed.  Sprinkle  irrigation was used during seed germination in the autumn, and drip irrigation through the winter and spring.  Spinach was seeded with two passes of a 4-row pinpoint seeder that distributes 1 seed per inch in a row, with 2.25 inches between rows.  From December through February spinach was protected in the unheated high tunnels under a sheet of spunbonded polyester fabric (Typar 580, Ken-Bar, Peabody, Mass.).
 

        
           Shortened day length and cooling night temperatures affected spinach growth so that days between planting and first harvest were vastly different for spinach planted in early October compared to November (Fig. 1 and Fig. 2).  Autumn and winter harvests were possible with spinach planted in early October.  March harvest was possible with spinach planted at all dates in October and November.  Harvest dates for each high tunnel planting are indicated in Table 1.

Table 1.  Harvest dates of spinach planted in high tunnels in autumn 2005.

Figure 1. Intervals from planting until harvest of Avenger spinach.  Successive harvests are indicated by change in crosshatch patterns, with final harvest on 27 March 2006.

Figure 2. Intervals from planting until harvest of PVO172 spinach.  Successive harvests are indicated by change incrosshatch patterns, with final harvest on 27 March 2006.

Table 2.  Analysis of variance of the effects of planting date and management (conventional or organic) on total spinach yield in trials at Olathe, Kansas, planted in autumn 2005.

          Planting date affected total harvest yield mainly because of differences in autumn harvests.  It can probably be assumed that the crops with reduced spring yield were less established or had fewer reserves, going into winter.  Spinach planted in November was not harvestable until February.  Yield data are presented for Avenger (Fig. 3) and PVO172 (Fig. 4) in organic and conventionally managed plots.  Yield in organic and conventional high tunnels differed somewhat in individual harvests, but there was not an overall statistically significant difference between spinach yield in organic and conventionally managed high tunnels (Table 2).  Harvest trends due to planting date were similar in organic and conventionally managed high tunnels (i.e. no interaction between planting date and management).

          Early planting of Avenger spinach increased the cumulative harvest.  Avenger spinach planted on 6 and 11 October had significantly greater total yield than spinach planted on 17 November in organically managed high tunnels, and greater than that planted on 8 and 17 November in conventionally managed high tunnels (Figure 3). 

         Spring harvest may be reduced in spinach planted after the first week of November. Delayed plantings of Avenger spinach in the month of November in conventional high tunnels resulted in significant yield reduction of 17 November plantings compared to 3 and 8 November plantings, but in organic high tunnels the February-March yield was not significantly reduced (statistical data not shown). In conventionally managed high tunnels, spinach planted on 11 and 25 October did not have a reduced total yield compared to that planted on 6 October (Figure 3), but because of differences in harvest dates the spring yield was statistically similar to the low yields of the 17 November planting, and less than the 3 and 6 November plantings (statistical data not shown). October planting date did not affect spring harvest (February-March) in organic high tunnels.

Figure 3. Fresh weight yield of successive harvests of Avenger spinach planted on six dates in the autumn of 2005, and harvested through March 2006, in conventional and organicially managed high tunnels at Olathe, Kansas. Columns are stacked with final harvest on top. Total harvest is not significantly different (a=0.05) for planting dates with the same alphabetic letter indicated above the column.

Figure 4. Fresh weight yield of successive harvests of PVO172 spinach planted on six dates in the autumn of 2005, and harvested through March 2006, in conventional and organicially managed high tunnels at Olathe, Kansas. Columns are stacked with final harvest on top. Total harvest is not significantly different (a=0.05) for planting dates with the same alphabetic letter indicated above the column.

Early planting of PVO172 spinach resulted in greater yields. PVO172 spinach planted on 6 October had significantly greater total yield than that planted in November in conventional and organic managed high tunnels (Fig. 3). Plantings on 11 October in conventional high tunnels resulted in significantly greater yield than all November plantings, and in greater yield than the 8 and 17 November plantings in organic high tunnels (Fig. 3). Each week of planting delay in November reduced yield. Spinach planted on 17 November produced significantly less spinach for spring harvest (February-March) than earlier plantings (statistical data not shown). All other plantings in organic and conventionally managed high tunnels had statistically similar spring spinach yields (statistical data not shown).
 

Spinach bed in high tunnels at Olathe, Kansas on January 5, 2006, showing development of weekly plantings from October 5 through November 17.
 These views are from opposite ends of the same bed.

         
          Over wintering of spinach in unheated high tunnels was successfully demonstrated with two cultivars, Avenger and PVO172.  Planting date affected harvest dates and total yield.  October planted spinach can be harvested in the winter without significant loss of spring yield.  Harvest trends of the two cultivars were similarly affected by planting date.  There was a point in mid-November where late planting significantly reduced spring yield for both cultivars.  Spinach planted at staggered dates through October in unheated high tunnels at Olathe, Kansas, produced spinach for harvest from November through March.  Future studies should evaluate both earlier and later planting date effect on winter and spring production.  Temperatures should be monitored so that results can be interpreted with respect to heat unit accumulation and day length.