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
How deep do I plant ageratum?
You will want to work the soil and dig a hole a little larger then the pot your plant is in. Keep the plant at about the same level as it was in the pot. Fill the soil in around the plant and press the soil in around it, firmly but not to tightly. Water plants in after you have planted them.
Here are a few links for you.
Will they grow back next year?
They will not. Ageratum are annual flowers. Here is an article with more information if you'd like: https://www.gardeningknowhow.com/ornamental/flowers/ageratum/growing-ageratum-flowers.htm
I usually sprinkle them on top of the soil but the weather has been so different this year I don't know when to plant. I live in Southern Maryland not terribly far from the Chesapeake Bay, the Patuxent River and the Potomac River. Our weather tends to move up from the Northern Neck of Virginia. How much sun does this variety require?
It is recommended to start them indoors about 6 to 8 weeks before the last frost date. Barely press the seeds into the soil surface. Light is required for germination. They should emerge in 7 to 10 days at 70 degrees F. When 2 inches tall, thin seedlings. Transplant outside after the last frost date in full sun to part shade in well-drained soil. This variety does not need pinching. Here are general growing tips: