Although they seem to be very different vegetables, onions and potatoes share in common that they store energy underground in a bulb or tuber that the plant can draw from during times of dormancy. For this reason, onions and potatoes are rarely or never started from seed, but rather from onion sets and seed potatoes, small pieces of the plant that will develop underground into an edible vegetable.
Onions and potatoes both function in the same way as flower bulbs. They do not form the root of the plant, but rather store energy that the plant uses to survive the winter, regenerating in the spring. Onion sets are small, immature onion bulbs that, when planted, grow a green top and develop a larger bulb. Seed potatoes are pieces cut from a potato, each containing at least one eye. The eyes develop into new plants.
One of the benefits of planting onion sets and seed potatoes is the durability of the vegetables they produce as compared to other vegetable crops. Because onions and potatoes are intended to lie dormant for long durations, they also preserve very well under the right conditions. According to the University of Illinois Extension, both onions and potatoes harvested in the fall will keep throughout the window, if properly stored, letting you enjoy the fruits of your garden year-round.
Although there are many types of onions available, according to the University of Illinois Extension, they tend to be sold by color--white, yellow or red--rather than variety. Large onion sets are harvested early as green onions, while smaller sets are allowed to develop into large bulbs. Potatoes also come in multiple varieties. Because of the prevalence of disease among potato crops, you should only plant seed potatoes that have been certified as disease-free.
As soon as the soil has thawed enough that you can work it, you can plant onion sets. Onion sets produce both green onions and bulb onions, depending on when they are harvested. Plant sets 1 inch deep, spacing them 1 inch apart for green onions and 4 inches apart for bulb onions. When the tops are 6 inches tall, sets may be pulled for use as green onions. When the tops fall over in late summer, bulb onions are ready for harvesting.
Potatoes are also planted early, in March or early April, according to the University of Illinois Extension. However, varieties of potatoes exist that thrive with later plantings, and later harvest are more suitable for winter storage. Plant potatoes in a furrow 3 inches deep, spaced about a foot apart. When the vines die back, gently dig them from the soil with a shovel or garden fork. Potatoes harvested in late summer or early autumn will keep throughout the winter if stored in a dry, dark place.