Start pearl onions in mid-spring just before last frost. Onions do best in starting temperatures of 55 to 60 degrees F and in warmer temperatures during growth and harvest. Pearl onion cultivars feature short maturity dates of 50 to 70 days, so fall frost is not an immediate concern to planting dates.
Put the pearl onions in a site with full sun exposure for 6 to 8 hours every day and quick drainage. Like all root crops, pearl onions rot in standing water. They need bright sun for effective growth.
Till the soil to give pearl onions a loose, airy foundation for easy growth. Dig the top 10 inches of soil and remove rocks, then incorporate 4 to 5 inches of organic matter like compost or wood chips. Onions cannot grow in tight, poor soil. Turn 5-10-10 fertilizer into the soil for quick rooting and growth.
Plant pearl onion seeds 1/4 to 1/2 inch deep with 1/2 to 1 inch of space in the row. Leave 2 to 3 inches of space between multiple rows for these miniature cultivars. Water the entire garden with 2 inches of water. Use a light spray function on the hose to avoid displacing seeds.
Water the onion plot with 2 inches of water every week, and lay organic mulch on the soil around the plants to discourage weeds. Thin the seedlings to every 2 inches in the row when they sprout so the onions have more room to grow.
Harvest pearl onions when their foliage dries and collapses. Pull up the plants to harvest the onions on their roots.