Shrubs require a much larger planting area to accommodate their roots than smaller perennials, so container-grown shrubs do particularly well in wine barrels rather than smaller containers. Most garden centers sell wine half-barrels ready for planting, but you can cut a full-sized wine barrel in half to make two planters. The most important consideration is that wine barrels are made of wood, which can rot from exposure to moisture in soil. Choose compact shrub varieties such as "Mathilda Gutges" hydrangea (Hydrangea macrophylla "Mathilda Gutges") or a dwarf, pyramidal conifer such as "Wilma Goldcrest" Monterey cypress (Cupressus macrocarpa "Wilma Goldcrest").
Stand the barrel upright and cut it in half through the midsection. Wrap a chalk line around the barrel and snap the line to mark the cut line. Use a circular saw to cut through the barrel. You can use both halves for planting shrubs. Skip this step if you purchased a half-barrel ready for planting.
Flip the half-barrel bottom side up. Drill three or four evenly spaced, 3/4-inch-diameter drainage holes through the bottom of the planter.
Line the inside of the barrel with black plastic to protect the wood from rot. Poke holes in the plastic lined up with the drainage holes drilled in the bottom of the barrel.
Cut a piece of plastic or wire mesh, or a woven fabric such as burlap, to fit the bottom of the barrel. Place the piece in the bottom of the barrel to prevent soil from falling out through the drainage holes. Staple the soil screen in place, using a staple gun and wood staples.
Set the barrel in place where you want to grow the shrub so you don't have to move a heavy planter filled with soil and a shrub; follow the sunlight needs for the plant variety. It helps to set the planter on a few bricks or stones so water can drain freely through the drainage holes without pooling up on the planter bottom.
Measure the height of the wine barrel and the height of the root ball. Add well-draining potting soil to the bottom of the barrel planter so that the root ball will rest about 2 to 3 inches from the rim of the planter. Wine barrels typically measure 36 to 38 inches tall, so a half-barrel measures 18 to 19 inches tall. If the half-barrel is 18 inches tall and the root ball measures 12 inches tall, add 3 to 4 inches of potting mix to the bottom of the planter.
Set the shrub in the wine barrel planter. Loosen the roots around the outside of the root ball so they spread more easily through the potting mix. Adjust the potting mix beneath the root ball so the shrub rests level in the planter.
Fill in the sides of the planter around the root ball with more potting soil; the potting soil should rest even with the top of the root ball. Pack the soil gently with your hands just to remove large soil pockets. Add more soil to bring the soil level back up to the root ball, if needed.
Spread a 2- to 3-inch layer of bark chip mulch around the shrub to fill the wine barrel to the top edge. Leave a few inches around the shrub stem or trunk free of mulch to avoid rot and infestation.
Water the soil in the wine barrel planter deeply until water begins draining through the bottom of the planter. Supplement 1 gallon of the water with an all-purpose fertilizer, such as 10-10-10, mixed at a rate of 1 tablespoon per 1 gallon of water. Follow the watering frequency required for the type of shrub you plant. Some shrubs require you to let the soil dry slightly between waterings, while others prefer a consistently moist or wet soil.