The following is written based on my experience with growing butternut squash in the high deserts of southern Arizona, latitude 31.5 N, elevation 4600 ft. where we have about 6” of rain July 1-September 15 and an alkaline mineral soil initially of pH 8.5 . If you grow them elsewhere you will need 90 to 100 frost free days.
Butternut squash prefer a slightly acid soil (pH 6.0-6.8) are heavy feeders that require multiple applications of fertilizer through the growing season. There is no need to plow an entire field to prepare for butternut squash as you will be growing them in relatively small mounds and the vines will spread out to a diameter of eight feet from the mound over unimproved soil. If you have a good loam soil (40% sand, 40% silt, 20% clay) all you need to add is a lot of organic matter and a bit of fertilizer. If you have a clay soil, add fine sand/silt. In the fall prepare the soil by marking out where you will plant. Dig into the soil when it is moist, not wet, to a depth of 18” and 18” diameter(2.6 cubic feet) one-half cubic foot (about five gallons) of uncomposted manure, 8 tablespoons of fertilizer (16-16-16) and five tablespoons of soil sulfur (to reduce pH to 6.5 from 8.5 like mine). Elsewise, use 1 tablespoon of sulfur per cubic foot of soil to reduce the pH by one unit. Soak with water to allow the fertilizer to dissolve and organic matter to rot over winter. Add more water every few weeks if you live in a dry climate. If you failed to prepare in the fall you can still plant in the spring but you will need to use composted manure and the fertilizer (16-20-0) may burn the roots unless you add it at a depth of 3” below the seeds. To make the digging to 18” and mixing the amendments easier, place half the soil in a wheelbarrow with half the amendments and mix those there while adding the other half of the amendments to the hole as you dig to 18”. My soil stops at 12” when I hit caliche but I still get a good crop.
In the late spring (May 15 here) form mounds and a bury a water pail (see last page for description) to 1” depth in the center of the mound. Plant three seeds at 5” radius from the center of the mound at a depth of 1/2” to 1”. To locate the seeds, mark a circle of 12” diameter by using the open end of a five gallon pail or use the suggested screened shelter (see last page for description) and place the seeds 1” to the inside of that mark and evenly spaced around the perimeter. Add water to the pan and over the entire mound and keep the ground moist until the seeds sprout. Cover the seeds with the screened shelter to protect from birds and rodents. To speed germination cover the shelter with translucent plastic. Remove the plastic after the seeds have risen. If all the seeds have not risen in two weeks replant those not risen. Replace the wire shelter. Once the plants are too large for the shelter remove it.
After the plants are developed with foot long runners, allow the first two inches of soil to dry before adding a total of 1” to 2” of water per week. This is enough so the soil will be moist at one foot (one gallon of water spread over 18” diameter is 0.9” of water). The water amount and frequency will depend on the temperature. During the hottest days you may need to apply one quart of water every day to reduce wilting. Allowing a plant to wilt will damage the fruits or they will not set fruit at all. If you can provide shade in the afternoon during the hottest time of the year do so. If you have many mounds, watering is easiest with a simple irrigation system. If you use such a system, water at a cool time of day so the water coming out onto the plant roots is cool. If you water during the brightest time of the day the water may exit at a temperature such that it scalds the roots even if the air temperature is moderate. If you scoop the common soil away from the outside of the mound to a distance of three to four feet you will form water harvesting swales that capture rainwater. This should be done before the vines interfere. Continue to water if there is not sufficient rain. As the vines spread outward it is possible to encourage additional roots to develop along the runners by pinning down the vines at the joint points. A light sprinkle of fertilizer in the swale area will improve its fertility. Add one-quarter tablespoon of fertilizer to the water pail every two weeks after the first blossoms appear. Each fall add more organic material and if you use uncomposted organic matter use triple-sixteen fertilizer.
When the vines reach four feet in length pinch off the ends to encourage lateral vines to develop. If you want many fruits harvested gradually over several months you can plant seeds every other week, in additional mounds up to mid July (mid August assuming warm weather to November 15). Space mounds at eight feet. Once a vine has three to five fruits (depends on the variety), pinch out any new blossoms or fruits to force food into the remaining fruit.
If you do not have good soil you can grow a single squash vine in a five gallon pail with drain holes in the bottom. The bucket sides need to be sheltered from the sun otherwise the soil may overheat. Use a high quality potting soil that has a slow release fertilizer.
Harvest the fruits at 90 to 100 days when a fingernail will not indent the skin or the stem begins to turn brown. Harvest the fruit by cutting the stem, don't break off the stem from the fruit. Remove all fruits before a frost as it will damage them and the shelf life will be nil. Separate all damaged fruits from the others. It is ideal to cure the fruit in the field by storing at 80-85 Fahrenheit and 80% relative humidity for 10-14 days in a plastic shelter to retain the moisture. Long term storage should be at 50-55 F and 50-70% humidity and will keep at least three months. If not cured, let the fruits stand on the shelf for at least two weeks to come to full sweetness before cooking.
Vine borer worms are the greatest pest. Look for damage to the vine indicated by a tan color sawdust where the worm has entered. Look for damage every couple of days starting in late June. The only thing I can recommend to kill the worm is to use a needle to pierce the worm hidden within the vine. Pierce along the length multiple times one inch on other side of the damage area. I am not sure what type of insecticide to recommend but if you use one apply it in the evening when the blossoms are closed and the bees are in their hives. It is recommended to dig new mounds every year as the vine borer worms will lay their eggs in the soil and that pest problem will increase. You can reuse the old soil if you pour into each mound one gallon of non-sudsy ammonia and cover with a patch of plastic to slow its evaporation. The ammonia will fertilize the soil and sterilize it of parasites.
Powdery mildew fungus will attack the vine leaves in August. Spraying with fungicide is only slightly effective and very difficult to do as both the tops and undersides of leaves need to be sprayed. As the old leaves are attacked and die, new leaves will begin to sprout and the additional roots along the length of the vine will help to sustain the plant. Hopefully the fruits will be well along in development before the fungus destroys the vines.
If you have rabbits (and you will) surround the squash patch with at least 18” tall 1” mesh chicken wire with at least 4” draped on the ground outside the enclosure. A small enclosure needs only half inch rebar to support it and bailing wire to attach the screen to the rebar. If you have havalina or steers you will need to use T-posts and stands of barbed wire. If you use 24” wide chicken wire, run one strand near the ground, another strand at 20” from the ground to support the top of the chicken wire, a third strand at 29” (for havalina) another strand at 38” and at least one more strand at 48” (0”, 20”, 29”, 38”, 48”). If you use 18” chicken wire set the strands a 0”, 14”, 22”, 34”, 48”. Attach the chicken wire to the strands with bailing wire every foot. Otherwise, use field fencing with chicken wire at the bottom.
Weeds can be partly controlled by watering the area where you will plant in the early spring to encourage the seeds to sprout which can then be sprayed with glyphosate herbicide (typical brand name Roundup). You can continue to spray until the vines seeds have sprouted. If you spray beyond that time take great care to keep the spray from drifting onto the young plants. After that you will need to pull or use a hoe to control weeds.
A 12” diameter shelter can be made with 37 ½” of plastic edging overlapped by 1” and held together with three short gold screws. Cover with window screen 14” x 14” (consider using aluminum for durability as creatures can eat their way through fiberglass screen) and hold in place with heavy 3/8“ staples at four places around the perimeter. Bend the staples over. I suggest using the solid plastic edging 4 ¾” X 1/8”.
Plastic water pails can be made from the bottoms of one gallon milk or bleach containers to help maintain a good water well and distribute the water. Cut the bottom off the container so the sides are 1 1/2” tall. Puncture the bottom with a nail or thumbtack in a dozen or more places so the water will drain into the soil. Set the pail at the center of the mound and sink 1” deep into the ground. A 6” x 6” x 1 1/2” pail will hold about one pint.