A clump of milkweed can reach a height of up to 6 feet (although 5 feet is more typical), and measure about 3 feet across. For that reason, you should space apart plants sufficiently. Some types of milkweed are bulkier than others; swamp milkweed, for example, needs about 30-36 inches of space between plants, whereas 18 inches is adequate for most other types of milkweed.