Spinach

Introduction

Spinach 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°F to 60°F with day length of approximately 12 hours. Winter spinach production in unheated hightunnels 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 well under high temperature and long day 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. 

Spinach Varieties Type Season Resistances
Acadia semi-savoy SF DM1-13, 15, 16
Carmel semi-savoy SF DM1-11, 13
Corvair smooth W DM1-11, 13
Emporer semi-savoy SFW DM1-1-
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 DW DM1-11
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 Kitten smooth SFW DM1-13, 15
Reflect semi-savoy SFW DM1-11
Regiment semi-savoy SFW DM1-7, 11
Renegade smooth SFW DM1-7, CMV
Samish savoy FW DM1-4
Santorini semi-savoy SFW DM1-7
Seaside smooth S DM1-12, 14
Shelby smooth SF DM1-13, 15, 16
Space smooth FWS DM1-3, 5, 6, 8, 11, 12, CLS
Unipak 151 semi-savoy SFW DM1-4, CMV
Whale smooth SF DM1-7
Winter Bloomsdale OP savoy W CMV
Woodpecker smooth SF DM1-15
Yukon semi-savoy W DM1-12, 14, 15

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

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

 

Soil Fertility

Apply lime according to soil test results to maintain soil pH at 6.5 to 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, make a topical application of 10 to 15 lb magnesium sulfate (Epsom salts) in 100 gal water. 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.

Promote efficient nitrogen use by sidedressing nitrogen when crop need is apparent. In winter production, there may be 6 contiguous months of harvest from one crop, and therefore sidedressing may be necessary, though N use efficiency of crops is less in cold temperatures. Avoid putting fertilizer directly onto crop foliage.

Less nitrogen fertilizer will be needed if manure or legume sod was plowed down (see Table 1, Nitrogen Credits from Manure and Table 2, Nitrogen Credits from Previous Crops).

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
*SEE PLANT NUTRIENTS FOR INFORMATION ON NUTRIENT MANAGEMENT AND APPLICATION.

Planting

Seed will germinate at soil temperatures of 32°F to 60°F. Soil temperatures above 700F will result in poor germination. Another cause for poor germination is salt accumulation in the top 0-2” 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.

For direct-seeded spinach, the desired plant stand is 6 to 8 plants per foot of row and 12" between rows. This requires 8 to 10 lb of seed per acre (1/2 to 1 oz per 100 feet of row). After preparing a stale seedbed, seeds may also be broadcast for a denser planting to help control weeds. During dry conditions, irrigation may be necessary to germinate seeds. Seed 0.25" to 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. Seedlings are typically ready for transplant approximately three-weeks after initial seeding. Using this method, desired plant spacing is 3-4 plants per square foot, or an equivalent plant spacing of 6”x6”. This method is particularly useful if the site is not yet prepared for the next crop, and for a head-start in production.

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 drain early and warm a little quicker. 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 (1.25oz/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. Within high tunnels, plants should covered with a single or double layer of floating rowcover (>0.55oz/yd2) and/or a low tunnel.

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 (>70F). 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 – 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. However, cool humid conditions can result in downy mildew, and thus it is important to chose varieties with downy mildew resistance for winter culture. 

In high tunnels, there are three common physiological disorders that may occur on winter-grown spinach. 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 managing 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 to 45 days after seeding, but can take much longer to mature during late fall and winter months. For winter production, earlier fall planting dates are associated with fewer days to harvest than late fall installation dates.

In summer and fall, spinach harvested early in the day and cooled immediately will have a much better shelf life (10 to 14 days). In the winter, frozen leaves can be damaged during harvest, so wait until later in the day after leaves have thawed.

Good yields for fresh market will range from 5 to 7 tons/A and 10 to 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 to 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.