DIY concrete planters are extremely adaptable in terms of style: Use fluted forms to fit with cottage-inspired garden furniture, or choose straight lines and geometric shapes for a more modernistic look. Plant with a few bulbs for early blooms in springtime; transition to colorful annuals in summer and autumn. Be sure to push your interior mold down far enough so the inside gives plants room to root. If you don't want to see the soil in the planter, use small rocks as a mulch to complement the concrete material.