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 year-round greens. 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.

Spinach Varieties
Variety Type Season Resistances
Acadia semi-savoy SF DM1-13, 15, 16
Auroch smooth SFW DM1-12, 14-16, 19
Carmel semi-savoy SF DM1-11, 13
Corvair smooth W DM1-11, 13
Emporer semi-savoy SFW DM1-10
Escalade semi-savoy SF DM1-14, 16
Flamingo Asian SFW DM1-11, 12, 13
Pigeon smooth W DM1-13
Gazelle smooth F DM1-13
Giant Winter OP semi-savoy FW DM1-11
Kolibri semi-savoy SFW DM1-9, 12-15,17
Kookaburra semi-savoy SF DM1-13
Marabu RZ semi-savoy SF DM1-10, CMV
Palco semi-savoy SF DM1-5, 8-9, 11-12, 14
Red Tabby smooth SFW DM1-9, 11-13
Reflect semi-savoy SFW DM1-11
Regiment semi-savoy SFW DM1-7, 11
Renegade smooth SFW DM1-7, CMV
Responder savoy W DM1-12, 14-16
Seaside smooth S DM1-12, 14
Shelby smooth SF DM1-13, 15, 16
Space smooth FWS DM1-3, 5, 6, 8, 11, 12
Sunangel semi-savoy SFW DM1-9, 11-17
Winter Bloomsdale OP savoy W CMV

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, there may be 6 contiguous months of harvest from one crop, therefore sidedressing may be necessary. Nitrogen use efficiency of crops is less in cold temperatures, so more frequent sidedressing of small amounts of N will achieve better results than fewer sidedresses 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 – it is recommended to 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).

PLANT NUTRIENT RECOMMENDATION ACCORDING TO SOIL TEST RESULTS FOR SPINACH
SPINACH NITROGEN (N) LBS PER ACRE PHOSPHORUS (P) LBS P2O5  PER ACRE    POTASSIUM (K) LBS K2O      PER ACRE
SOIL TEST RESULTS   VERY LOW LOW OPTIMUM ABOVE OPTIMUM VERY LOW LOW OPTIMUM ABOVE OPTIMUM
Broadcast and Incorporate 60-80 180 120 30-60 0-30 180 120 30-60 0
Sidedress 3-4 Weeks after Planting 30 0 0 0 0 0 0 0 0
TOTAL RECOMMENDED 90-110 180 120 30-60 0-30 180 120 30-60 0

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 heavily composted high tunnels. Priming the seed may improve germination. 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. 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. Winter production of spinach by New England growers is now quite popular. Stand-alone low tunnels equipped with heavyweight row cover (1.25 oz/yd2) and plastic (6mil) 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. 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. 

For overwintering spinach, particularly in northern areas, plants should be established by early-November. If direct seeding, a September seeding date is suggested. However, germination can be poor in tunnels with warm soil temperatures (>70° F). Transplanting seedlings instead of direct seeding can help mitigate this risk, and ensures a good stand for winter production.

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

Highest total yields (fall through spring) will be obtained by the earliest fall planting dates, primarily due to higher fall production. Spring-only yields (January to April) are less affected by fall planting date.

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 - tunnels may only need to be irrigated once or twice throughout the winter months when little growth is occurring. However, cool humid conditions can result in downy mildew, and thus it is important to choose varieties with downy mildew resistance for winter culture. 

In high tunnels, there are three common physiological disorders that may occur on winter-grown spinach. Although 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 secondary row covers or low tunnels inside high tunnels. 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 often forms natural leaf structures (glandular trichomes) on upper and lower leaf surfaces, which resemble tiny water droplets or insect eggs, but actually arise from the leaf surface on tiny stalks. These trichomes often cause concern, but are harmless.

Harvest

Spinach is usually harvested from 37-45 days after seeding, but can take much longer to mature during late fall and winter months. 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 in larger-scale production. 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, frozen leaves can be damaged during harvest, so delay harvest until later in the day after leaves have thawed. 

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, like carrots, when going directly to retail markets. 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.