When it comes to planting a vegetable garden, a gardener must keep several things in mind: season, location, soil, compatible plants and protection for the garden. All the hard work that went into the garden can be ruined quickly by an insect or worm invasion before or during vegetable production. Vegetables like carrots and onions attract specific pests -- carrot and onion flies -- that can damage both the plants and their crops. Discerning gardeners, though, know that carrots repel onion flies and onions repel carrot flies. A smart gardener plants these two vegetables together for mutual protection.
Choose the right location for the garden. Both carrots and onions are summer vegetables that require full sunshine for eight hours a day. Plant your garden in a location that gets complete drainage after any rain, as vegetables suffer in standing water. Plant carrots and onions together in the garden, and plant onions as a border around the garden to guard against other pests like rabbits and caterpillars.
Amend the soil before planting, with a mixture of half quick-draining soil and half organic compost. This mixture ensures both drainage and nutrition for the vegetable plants. Mix the amendment into the top 2 inches of soil, and remove any weeds or rocks from the site.
Plant carrot seeds starting in early April for a summer harvest. Sow the seeds directly into the soil at 1/4 inch deep, with spacing of 12 to 18 inches. Once the seedlings sprout, thin the planting so you have a carrot at every 2 to 3 inches. This gives the carrots adequate room to grow.
Plant onion seeds early in spring, after the ground thaws. Sow onion seeds into the garden at a depth of 1/2 inch and spacing of 3 inches. If you're planting rows of onions, plant them at 12 to 18 inches. Intersperse onions with the rest of the vegetable garden to protect all plants from pests.
Water the garden with 1 inch of water, and put both onions and carrots on a weekly schedule of that amount. Once seedlings emerge, spread an inch of organic mulch over the soil to conserve soil moisture and warmth for the root plants.
Carrots grow to maturity very quickly, and will provide a vegetable harvest in early summer. Once you've harvested your first round of carrots, replant for further protection and another carrot harvest.
Never use fresh manure with carrots, as it will burn and deform the plants and their root crop.