Soil preparation and mulch options

Few producers would want to spend a night in a lumpy, cold, wet bed. So why would they subject tender seedlings and young plants to the same environment? To get a field of pumpkins or other cucurbits off to a good start, a producer must offer a nice seedbed.

“The biggest thing is soil preparation,” says Bill Lamont Jr., professor of vegetable crops at Penn State University. “You have to be thinking ahead. Building and maintaining your soil is a challenge.”

Just as cold pockets in your bed at home make for an uncomfortable night, cold pockets under the plastic mulch used by many operations will make young seedlings mighty uncomfortable. Pockets are caused by lumpy or cloddy soil.

That said, a pumpkin producer might want to decide whether or not to plant pumpkins in a particular field at all. “We recommend that you not plant pumpkin more than two years in a row in one field, and then stay out of that field for three years at least,” says Ruth Hazzard, team leader for the vegetable program in the Department of Plant, Soil and Insect Sciences at the University of Massachusetts, Amherst.

This caveat comes because of Phytophthora capsici. A devastating disease of cucurbits and other vegetables, it causes a root, crown and fruit rot. Root and crown rot are particularly destructive because the entire plant collapses.

“Once a field is infected, it will not go away,” Hazzard warns.

She says producers should avoid planting pumpkins and other cucurbits in low spots. “Every field has a low area. That’s where you’ll see P. capsici,” Hazzard notes. She recommends planting corn or grass in low areas. Everywhere else, Hazzard recommends starting with deep-zone tillage – cutting a deep vertical shaft through the soil compaction layer.

This will help with both drainage and maintaining soil moisture between irrigation cycles. “You’ll build up better soil structure,” Hazzard explains. “It makes the soil more resilient.”

Ready for plastic

Gold Medal from Rupp Seeds is a large pumpkin with a dark color and long handle.. Photos courtesy of Rupp Seeds.

Gold Medal from Rupp Seeds is a large pumpkin with a dark color and long handle. Photos courtesy of Rupp Seeds.

“You need nice, tight contact and no air pockets,” Lamont says. A good bed is one where the plastic mulch will be in close contact with the soil everywhere. A standard practice for most Northeastern producers would be to create a 30-inch-wide bed that stands about 4 inches high. A 40-inch piece of plastic will cover the bed.

“You have to work the soil so you have good plastic-to-soil contact, otherwise you won’t get uniform warming,” Lamont explains. You don’t want a rough soil surface to create air space between the plastic mulch and the soil.

This problem is typically caused by working the soil when it’s too wet. While this is not a major problem on the sandy soils of the coastal plains and into Florida, it’s a major concern for producers on Northern loam or clay soils.

Lamont likes to see producers use a rototilling apparatus to work the soil into a fine structure. This ensures that there is plenty of loose material to build the bed and to fill in any dips or peaks.

A lumpy seedbed can also affect the drip irrigation system. “If your tube is pulled down with a shank, you will get a channel on the top of the bed,” Lamont says.

He suggests working up a little crown on the raised bed, which will allow water to shed off the mound. The crown will help with melon and tomato crops. “You don’t want a melon lying on the bed,” he adds.

Rupp Seeds' Gold Standard is a popular midsized pumpkin variety.

Rupp Seeds’ Gold Standard is a popular midsized pumpkin variety. Photo courtesy of Rupp Seeds.

Planting pumpkin

When the time comes to transplant a plant with a nice root ball on it, the soil cannot be too wet. Spacing matters too.

Ronald Morse, Virginia Tech professor emeritus of horticulture, did much of the seminal work on pumpkin seeding. His research shows that producers get optimum yield by using a semi-bush type plant, such as Gold Standard or Magic Lantern, and planting it on equilateral spacing (4 by 4 feet). This type of spacing produced 25 percent more marketable pumpkins than other spacings with the same area (for instance, 8 by 2 feet).

Popular midsized pumpkin varieties include Gold Standard, Magic Lantern and Merlin. Large, 25-pound varieties include Howden and Solid Gold. For even larger varieties (30 pounds or more), options include Gold Medal and Howden Biggie.

Note that there are no known pumpkin varieties that are resistant to P. capsici.

No-till might be a boon for market pumpkins, since the pumpkins will be cleaner and less susceptible to rot because they are not in direct contact with the bare ground. Hairy vetch and cereal grains can be used as cover crops for no-till pumpkin production.

While she doesn’t know of any growers who have gone completely to no-till, Hazzard says many commercial growers are interested in reduced tillage for crops like pumpkin and corn. Where the farm is growing under plastic, the bed must be properly prepared.

Regardless of the variety and cultivation method used, more commercial operations are looking to plastic mulches to help the crop along.

Know your plastics

“The workhorse is still black plastic,” Lamont says. “I like to use a 1-mil to 1.25-mil embossed material.” The embossed plastic typically has a diamond shape etched into it that allows the plastic to hold flat to the bed.

Three main colors – black, clear and white – are prevalent in commercial production today. White has largely been replaced by a co-extruded white-on-black.

Plastics make a big difference in the temperature of a seedbed, but only for a while. Once there is sufficient shading from the plant’s own leaves, any temperature increase or decrease due to plastics is neutralized, Lamont notes.

According to Penn State Extension information (, “Plastic mulches directly impact the microclimate around the plant by modifying the radiation budget (absorptivity versus reflectivity) of the surface and decreasing the soil water loss. The color of a mulch largely determines its energy-radiating behavior and its influence on the microclimate around a vegetable plant. Color affects the surface temperature of the mulch and the underlying soil temperature.

“The soil temperature under a plastic mulch depends on the thermal properties (reflectivity, absorptivity or transmittancy) of a particular material in relation to incoming solar radiation. … Black mulch absorbs most ultraviolet, visible and infrared wavelengths of incoming solar radiation and reradiates absorbed energy in the form of thermal radiation or long-wavelength infrared radiation.”

As long as black plastic has good contact with the soil surface, much of the energy it absorbs can be conducted to the soil. Compared to bare soil, daytime soil temperatures under black plastic mulch are “generally 5 degrees Fahrenheit higher at a 2-inch depth and 3 degrees Fahrenheit higher at a 4-inch depth,” according to Penn State Extension.

Clear plastic mulch doesn’t absorb much solar radiation, but transmits 85 to 95 percent, depending on how thick and opaque it is. Condensation on the underside allows clear plastic to retain heat that would be lost by bare soil through infrared radiation. “Daytime soil temperatures under clear plastic mulch are generally 8 to 14 degrees Fahrenheit higher at a 2-inch depth and 6 to 9 degrees Fahrenheit higher at a 4-inch depth compared to those of bare soil,” according to Penn State Extension. Clear plastic is used more frequently in cooler areas, such as the Northeast.

Penn State Extension notes: “White, co-extruded white-on-black or silver reflecting mulches can result in a slight decrease in soil temperature (-2 degrees Fahrenheit at 1-inch depth or -0.7 degree Fahrenheit at a 4-inch depth compared to bare soil), because they reflect back into the plant canopy most of the incoming solar radiation.” The effect is not season-long.

“Once the crop canopy covers in, all the temperatures even out,” Lamont explains. “Even with colored or metallized silver reflectives, the temperature evens out. It is only when the crop is small that you get the benefit.”

Other considerations

Weed control, even with plastics, is important. A pre-emergence material is good in most fields.

Depending on the number of planting holes, plastic mulch is still susceptible to weeds popping up through the holes. You’ll need to use herbicides, soil fumigants or solarization in conjunction with clear plastic mulch. White plastic mulch may also call for a fumigant or herbicide, depending on how opaque it is.

In some cases, the crop can outcompete the weeds without chemical help. In other cases, the weeds will win and the farmer will lose.

Despite your best efforts, Mother Nature sometimes deals us bad weather, and you’re almost forced to work the soil in less than optimal conditions.

When this happens, use that rototilling tool to break clods into small enough particles that the plastic and the young crop have a chance.

Lamont says that sometimes a producer can briefly run the drip irrigation system to help solidify the soil bed.

Drip irrigation uses less water – and uses it more frugally. Running fertilizer through the irrigation system is an effective way to get plant food where it’s needed.

“You are not wetting the crop canopy,” Lamont points out. This means the crop is under less disease pressure.

UMass recently changed its soil test recommendations from low, medium, high and very high to a ranking of very low, low, optimum and above optimum. It may seem subtle, but Hazzard explains that the idea of an “above optimum” level of phosphorus (P) and potassium (K) will serve as a warning to start mining the soil rather than applying more.

“We want to give a clear message that you do not want to add P and K if they already are in excess,” she says.

UMass recommends broadcasting part of any fertilizer application and placing the rest in a band on the bed. Typically, putting more fertilizer in the bed allows for concentrating the side-dressing based on soil nitrate testing.

Just because you get a good crop in the field doesn’t mean it will make it intact to market. Pumpkins keep well, but benefit from proper postharvest handling.

Any storage that allows good air movement will give the pumpkins a better shot at coming through well. Place pumpkins in bins or on racks. Ventilated storage assures good air movement and a crop that will make it to the consumer.

Intercrop Pumpkins

While intercropping pumpkins in a cornfield is a traditional practice, modern agriculture makes it a bit more of a management problem than it used to be.

It’s almost certain that the corn crop, typically sweet corn, will shade the crop and be harvested mechanically. When tractor wheels come into contact with large pumpkins in the field, the pumpkins will suffer.

Pumpkins are among the few vegetables that are rather shade-tolerant. Still, vegetable specialists like Bill Lamont Jr., professor of vegetable crops at Penn State University, prefer to see them grown separately.

Seeds are most often planted at a depth of 1 to 2 inches, depending on the soil moisture at the time of planting.

The Color of Mulch

Penn State researchers looked at various colors for mulches used at planting. Included were red, blue, yellow, gray and orange. Each has distinct optical characteristics and reflects different radiation patterns. This affects plant growth and development, as well as insect response to the plants.

According to Penn State, “Yellow, red and blue mulches increased green peach aphid populations, and yellow mulch attracted increased numbers of striped and spotted cucumber beetles and Colorado potato beetles. Yellow has long been used in greenhouses to monitor the population of insects. Mulches with a printed silver surface color have been shown to repel certain aphid species and reduce or delay the incidence of aphid-borne viruses in summer squash. Similar to a white mulch, the degree of opacity of a gray mulch may require an herbicide or fumigant to be used to prevent weed growth.”

Penn State found that blue and red mulches had a dramatic impact, “raising soil temperatures to 167 and 168 degrees Fahrenheit, respectively, at the 2-inch depth when the ambient air temperature was 104 degrees Fahrenheit.”