Of the various sizes of bark mulches, they are the best for weed control. Bark chunks are so coarse textured that they don't encourage a big growth spurt in the soil microorganism population that can tie up soil nutrients. Because the bulky pieces decay slowly, don't turn them under the soil, where they can create air pockets near plant roots. Bark chunks are a good ornamental mulch for trees and shrubs, or for covering the spaces between plants in a new planting. Spread to a depth of about 3-4 inches.
Wood chips are small pieces of wood about an inch in diameter.
The chips make a decorative mulch that breaks down faster than bark chunks, adding more organic matter to the soil. Wood chips are high in carbon, making soil organisms tie up nitrogen and other soil nutrients as they work to digest the chips. Freshly chipped wood needs to sit for a few months before you use it, since the tree might have contained natural substances toxic to young plants. Small chips may bounce onto sidewalks during a downpour. Spread to a depth of about 3-4 inches around trees and shrubs, or in the spaces between plants in a new planting.