Water Brussels sprouts once weekly so the plants receive about 1 inch of water each week. The plants may require two weekly waterings during dry weather so the soil remains moist.
Apply 1 tbsp. of an ammonium nitrate fertilizer to every 20 foot planting row when the Brussels sprout plant grows to a 12-inch height. Work the fertilizer into the soil 6 inches from the base of the plants and water after the application.
Remove the lower leaves on the stalk when the sprouts begin forming to speed maturity, if desired. Remove two additional leaves every week until the sprouts mature, but leave the topmost foliage on the stalk in place.
Cover the plants with floating row covers if cabbage worms or other pests are an issue in the garden. Brussels sprouts do not require pollination so they can remain covered for the entire growing season if necessary.
Harvest the mature sprouts when they reach a diameter of 1 to 2 inches. Cut the sprouts from the stalk with a sharp knife. The sprouts near the bottom of the stalk mature first and those at the top mature last.