Whiskey barrels make great planters, whether they are placed alone in the yard or used in a larger landscape design. The flowers are going to need special soil, though, and a little more attention than garden plants to keep them looking their best. When you go to the garden center to get the flowers, be sure they thrive in a full-sun exposure if the whiskey barrel is going to be in a sunny position.
Drainage and Soil
Adequate drainage is necessary for flowers to thrive when planted in a whiskey barrel. Drill four 1/2-inch holes spaced evenly apart in the bottom of the barrel before filling it with soil. The soil must be light and drain quickly. Garden soil in a whiskey barrel will not drain quickly enough. Purchase soil mix that is formulated for use in containers. Soil-based or peat-based potting mixes are fine. They should contain coarse sand, perlite or vermiculite for improved drainage, though, and may also contain ground tree bark. Set the whiskey barrel in place before filling it. They are very heavy after they are filled.
Flowering Plant Selection
Flowering perennials, annuals or a combination of the two can be planted in whiskey barrels. Trailing plants like trailing garden verbenas (Verbena x hybrida) and trailing lantanas (Lantana montevidensis) can be planted around the outer edge of the whiskey barrel and left to drape over the side. Trailing verbenas are hardy in U.S. Department of Agriculture plant hardiness zones 9 to 10 or 4 to 8, depending on the hybrid, and trailing lantanas are hardy in USDA zones 8 to 10. Verbena sap may irritate skin and lantana berries and foliage are poisonous when eaten, but both of these plants thrive in hot, direct sunlight. Upright plants like summer snapdragons (Angelonia angustifolia) and cockscomb (Celosia spp.), both annuals, can be planted in the center for bright, colorful flowers.
Planting the Flowers
The soil should be moistened thoroughly before planting the flowers. Mix the soil by hand or with a hand trowel while pouring water over it to make sure it is evenly moist. Plant the flowers far enough apart to allow them to reach their full size with 1 or 2 inches to spare. If the mature plant width is 6 inches, plant it 4 inches away from the other plants. Water them to settle the soil around their roots.
Water and Fertilizer
Slow-release fertilizer can be mixed into the top 6 to 8 inches of soil before planting. Usually 1 tablespoon is plenty, but follow the application rate recommended on the label. The flowers can also be fertilized with water-soluble fertilizer every two or three weeks mixed at a rate of 1 tablespoon per gallon of water. Use a balanced 10-10-10 or 8-8-8 fertilizer. The hot, direct sunshine and warm summer temperatures will cause the soil to dry quickly. Check the soil every day to see if it needs to be watered. Give them 2 to 3 gallons of water when the top inch of soil becomes dry. Organic mulch can be spread over the soil to reduce the need for frequent watering. Deadhead the flowers regularly to keep them looking their best all summer.