Asparagus: The vegetable is usually planted in mid-April or early May when soil temperatures are around 50-60°F. It prefers full sun (six to eight hours) in a soil with good drainage and a slightly alkaline soil pH, which is great for southern Wisconsin, since most of our soils are alkaline. If you have sandy soil, I suggest getting a soil test, since some sandy soils are more acid. You can get one through the UW Soils Lab. They are located in Madison. Contact them for a soil testing kit at 608-262-4364 or visit their website at https://uwlab.soils.wisc.edu/.
Asparagus is planted as 1-year-old “crowns” (bare-root plants with buds at the top) in trenches about 6 inches deep and wide enough to accommodate the full length of the roots when spread out, plus a couple of more inches to give them extra space to spread. Space plants about 18 inches apart. Fill in the trench with soil; research indicates it can all be done at once instead of a little at a time. If planting more than one row, space the rows 4 feet apart.
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Male plants yield 1.5 to 2 times as many spears as female cultivars since they don’t expend energy on producing fruits. Some popular male cultivars are “Jersey Knight,” “Jersey Prince,” “Jersey Gem” or the Canadian hybrid “Teissen.”
Don’t harvest spears the first year. You can harvest lightly the second year; research shows it stimulates more spears to form. Harvest when spears are 8 to 10 inches tall. You can snap them off near the base. The full harvest season is about six weeks; stop when spears are about pencil diameter. Leave at least some ferns standing in fall (you can cut them back about halfway) as research has shown this helps protect the crowns over winter.
Vegetable seeding: It should be safe around April 15 or so to plant seeds of certain cool-season crops, as well as onion sets and seed potatoes, directly outside in your garden beds if the soil has been worked up and is ready for planting. You may also now plant out seedlings of spinach, chard, mustards, kale and lettuce that you started inside, but you will need to cover them if frost is predicted. Seeds of crops like carrots, peas, turnips, radishes and beets should be planted outside instead of started indoors.
If you plant too early, or in cold wet soil, seeds are likely to rot instead of germinating. That’s where an inexpensive soil thermometer that you can purchase at your local garden center or online can be really handy, because Mother Nature does not always run on calendar dates. Even though the date may seem right, the soil temperatures may not be.
Seed potatoes need a minimum soil temperature of 40°F to sprout. Beets, carrots, radishes, lettuce and spinach all need a minimum soil temperature of 45°F to germinate. Snow peas and sugar snap peas need a minimum temperature of 50°F to germinate. Temperatures are likely to fluctuate a lot for a while yet, so even if it is very warm for several days, remember that the weather often self-corrects and becomes cold again, so watch the weather reports for frost predictions.
— Lisa Johnson, Dane County
UW-Extension horticulture educator