A quick dahlia grow guide for success: 1. Plant after your last frost, when soil temps are around 60 degrees F. Choose well draining soil and a location that gets plenty of sun. If you live in an extra hot climate though, planting where they’ll get some late afternoon shade is ideal. 12 inches apart often works well for spacing. 2. Wait to water until the tubers sprout above ground, unless you have very dry soil. In that case, make sure it gets a light watering so that your soil is lightly damp.