Spinach

Introduction

Spinach (Spinacia oleracea) is a hardy cool weather crop, grown for use as a cooked green vegetable or for salad greens. Temperature for optimum production and high quality is 55-60°F with day length of approximately 12 hours. Winter spinach production in unheated high tunnels has grown in New England to supply greens year-round. Under certain conditions, spinach will bolt (develop a seed stalk and flower), reducing quality. During summer months, high temperatures and long days will result in bolting. For spinach plants overwintered in high tunnels, bolting may occur in early spring as days lengthen. The tendency to bolt varies with the cultivar, some being more resistant than others. Growers who want greens in summer should consider beet greens and/or Swiss chard as substitutes, since they produce better under high temperature and long day conditions. Malabar spinach is not related to true spinach; it is a vining spinach-like crop native to tropical Asia and is well-adapted to growing in summer New England conditions.

Types and Varieties

There are two main types of spinach: smooth leaf and savoy (crinkled leaf). Both grow equally well and are marketed similarly, but the savoy type, because of its crinkled leaf, is more difficult to clean. Asian leaf types are relatively smooth with pointed leaves. Varieties best suited for winter production are often the fastest-growing varieties.

OP = open-pollinated, Seasons: S = spring, F = fall, W = winter. 

Resistant or tolerant to: DM: Downy Mildew (races indicated), CMV: Cucumber Mosaic Virus

Soil Fertility

Apply lime according to soil test results to maintain soil pH at 6.5-6.8. Soils with low pH will result in slow growth and chlorotic leaves.

Because of sensitivity to magnesium deficiency, older spinach leaves may tend to show yellow color similar to that caused by nitrogen deficiency or downy mildew. Low levels of magnesium in the soil can be corrected by using high magnesium lime (dolomitic) or by adding magnesium to the fertilizer. Do not automatically apply more nitrogen to try to develop the desired deep green color. Rather, fertilize with magnesium according to pre-plant soil tests and, if needed, test again mid-season and make a topical application of 10-15 lb magnesium sulfate (Epsom salts) in 100 gal water to correct any magnesium deficiency. Spray to wet the foliage.

If magnesium was deficient, you will see a color change to dark green overnight. Spinach will accumulate excess nitrates if nitrogen is used in an attempt to induce green color. It is always best to check for magnesium problems before applying extra nitrogen if plants have chlorotic pale green color or yellow lower leaves.

In winter high tunnel production, a single crop may be harvested for six months and therefore sidedressing may be necessary. Nitrogen use efficiency of crops is lower in cold temperatures, so more frequent sidedressing of small amounts of N will achieve better results than fewer applications of larger amounts of N. Avoid putting fertilizer directly onto crop foliage. The need for sidedressing will be influenced by pre-plant levels of N in the soil, which can vary widely between farms and high tunnels based on the soil nutrients remaining after the summer crop. Available N levels can be monitored mid-season using pre-sidedress nitrate tests. Sidedress with N if nitrate levels drop below 30 ppm.

Less nitrogen fertilizer will be needed if manure or legume sod was plowed down (see Table 1 and Table 2).

Planting

Seed will germinate at soil temperatures of 32-60°F. Soil temperatures above 70°F will result in poor germination. Another cause for poor germination is salt accumulation in the top 2" of high tunnel soil. Priming the seed (i.e. soaking he seed in water or another solution to kickstart the germination process prior to seeding) may improve germination, though there is no research-based seed priming protocol specifically designed for spinach. Spinach seed is short-lived and susceptible to damping-off. For good stands, start with new, fungicide-treated seed each year. Taking measures to prepare uniform, well-drained beds and to provide even irrigation can reduce variations in soil moisture that can lead to damping off as well.

Spinach can be direct-seeded quite densely, at up to 3,000,000 seeds per acre (70 seeds per sq. ft.). Target harvest size may affect seeding density, with “teenage” or larger-sized spinach seeded less densely and baby-leaf spinach seeded at the higher density. Harvest method may also affect preferred seeding density. Growers harvesting by clear-cutting, either by hand or mechanically, may prefer denser seeding rates, whereas those harvesting lower leaves only may prefer wider spacing. After preparing a stale seedbed, denser seeding may help to control weeds. During dry conditions, irrigation may be necessary to germinate seeds. Seed 0.25-0.5" deep depending on soil moisture and temperature. Deeper planting is suggested in a warm, dry soil. Growers should attempt to seed to a stand as thinning is generally not recommended. 

Spinach can also be germinated in seed trays and transplanted at the desired site. This method is particularly useful if the site is not yet prepared for the next crop, and to get a head-start in production. Seedlings are typically ready for transplant approximately three weeks after initial seeding. Again, plant spacing can vary based on target harvest size and method.
 

Field Culture

Main season (spring and fall). Spinach can be seeded in the spring as soon as the soil can be worked. Sandy soils are generally preferred because they warm earlier in the spring. Two main crops are generally grown, one in the spring, another in late summer, seeded about 6 weeks before the average first frost.

Winter Season. Winter production of spinach by New England growers is now quite common. Stand-alone low tunnels equipped with heavyweight row cover (1.25 oz/yd²) and plastic (6 mil) result in good winter survival of spinach and allow for separate fall and spring harvests. However, accessing plants during winter months when the ground is frozen is challenging, and leaves may not be saleable during this this period. High tunnel production permits winter access and provides additional insulation, and can result in winter-long harvests if plants are established by late fall. If direct-seeding, a September seeding date is suggested. Higher total yields (fall through spring) will be obtained by the earliest fall planting dates, primarily due to higher fall production. Spring-only yields are less affected by fall planting date.

Row cover is not required for winter high tunnel spinach production, as spinach is cold- and frost-tolerant, but the use of row cover can speed up growth. 

Plant growth slows with less than 10 hours of light per day starting around November 10 in Southern RI and October 30 in Northern ME. Aim to have 4-5 true leaves on plants before you reach this point.

It is important to ensure adequate moisture is available for spinach during the fall, winter, and spring months, either by using drip or overhead irrigation. The majority of irrigation is needed in fall and spring. lettle irrigation is needed in winter months when crop growth is slow. 

There are three common physiological disorders that may occur on winter-grown spinach. Spinach can withstand frost well, as long as leaves are not handled when frozen; freezing damage can kill and brown leaf tissue. This can be minimized by using row cover. Oedema results when water pressure causes cells to burst, resulting in scab-like calluses on the leaves. This can be minimized by limiting irrigation and maintaining low relative humidity as temperatures drop and growth slows. Lastly, under winter and spring conditions, spinach leaves often develop glandular trichomes on upper and lower leaf surfaces, which resemble tiny water droplets or insect eggs. They are more common on young leaves. These trichomes are naturally occuring structures produced by the spinach leaves and are harmless.

Harvest

Spinach is usually harvested 37-45 days after seeding, but can take much longer to mature during late fall and winter. Harvesting can be done by hand, either by clipping mature leaves and leaving young leaves uncut, or by clear-cutting just above the growing point. Walk-behind and tractor-mounted mechanical harvesters can be used. Regrowth quality varies by harvest method and the growth habit of the cultivar. 

In summer and fall, harvest early in the day and cool immediately to 32°F to maximize shelf life (10-14 days). In the winter, wait until leaves have thawed before harvesting, as handling frosted leaves will cause damage. 

Good yields for fresh market will range from 5-7 tons/A and 10-12 tons/A for processing. In winter high tunnel production, growers report producing 0.4–0.6 lbs/sq ft. Spinach is sometimes field packed loose into crates or cartons. Whole plants are sometimes bunched with roots trimmed off for retail sales. Package as bushel baskets or crates containing 20-25 lb, cartons or wire-bound crates with 2 dozen bunches each, or loose leaf 12 film bags (10 oz per bag) in a master carton. Winter spinach is often sold bagged in smaller quantities.