Cucumbers planted in rows should be 6 inches apart, and those planted in hills should have up to five seeds placed in a 1-foot diameter circle, according to the National Gardening Association. Hills should be up to 6 feet apart.
Rows of cucumbers should be thinned to allow 1 to 2 feet between plants once they are about 3 inches tall. Hills should be thinned to two seedlings once the plants have true leaves. Cucumbers grow quickly, and gardeners can plant a second set of seeds about four weeks after the first sowing to generate another harvest late in the season.