I have bought many plants and will put then in the ground today. How far apart do I plant them?
This is determined by the width, or spread, of the plants at maturity, and also by the look you wish to achieve. The size information should be on the plant tags, or available from the plant store. (I can't give it to you because there are a number of 'pearl' cultivars. If you wish to have the plants in a solid bank or line when mature, and you have plants that mature at 10" width, plant the no more than 10" apart, probably 8" will give you a better appearance. Here's more ageratum information: https://www.gardeningknowhow.com/ornamental/flowers/ageratum/growing-ageratum-flowers.htm