Walla Walla onions planted from sets mature earlier than other varieties of onions and can be harvested when the sweet onion bulbs are full sized and the tops fall over. Plant the sets in the spring as soon as the soil has dried enough not to stick to your gardening tools.pace the rows 12 to 24 inches apart. Place a Walla Walla onion set in the row every 3 to 4 inches. Cover the sets with soil, but do not firm the soil over the top of the sets.