Onions and potatoes are grown differently than most seed-raised garden plants. Onions can be grown from seed but most often are grown from sets because it's easier. Potatoes are grown from the eyes on a potato. One potato can produce several plants. Once the onion plants are started, the green tops can be harvested as they grow, but you'll have to wait until fall for full onion heads. Onion sets and potato eyes can be purchased from local nurseries for spring planting.
Grow onions and potatoes in an area that gets at least 6 hours of direct sunlight a day in the early spring after all danger of frost has passed.
Make sure the soil is loamy and well-drained. Mix compost into the top 6 inches of soil. Add 16-16-8 fertilizer evenly to the area where you plan to plant your onion sets. Level the garden surface with a rake. Remove any rocks from the area.
Plant marble-sized onion sets to prevent them from flowering too early in the growing season. Make sure your sets have not started to sprout.
Dig a row that is slightly hilly to plant the onion sets. Plant each set at a depth of 1-½ inches by placing the pointy end of the set up in the soil. Space each set 4 inches apart.
Start the potatoes about 1 foot from the onions. Cut 1- to 2-inch block shapes from the potato to remove each eye or bud. These pieces will be used for planting.
Dig a 3-½ inch deep trench for the potato buds. Place each bud 1 foot apart. Space each trench row about 2 feet apart. Cover the trenches with soil.
Add a 2-inch band of 10-20-20 fertilizer to each side of the potato rows. Place the band 3 inches from the plants on each side.
Water the garden to moisten the soil each day. Mound soil around the base of the plants when they are 6 inches tall. Create a 4-inch hill around the potato plants when they reach a height of 15 inches.