How to Grow Milkweed Plants: Plant milkweed about 18 inches apart; most varieties have rhizomatous roots that will quickly fill in the space between plants. There’s no need to fertilize milkweed plants, as they tolerate poor soils. Your biggest maintenance challenge may be in containing them. Milkweed spreads both via seeds and root systems (rhizomes) forming colonies. You may wish to remove the seed pods before they open. Otherwise, they will spread to distant corners of your yard (and beyond) thanks to the silky appendages that allow the seeds to waft on the slightest breeze.
Planting for Monarch Butterflies: If you are planting common milkweed to encourage monarch butterflies, create a small patch of milkweed that includes at least six plants. (Though again, some varieties will quickly spread from one to six on their own.)
Include a nearby water source for your butterflies; a birdbath or a large potting saucer filled with water will work fine. Planting other pollinator-friendly plants (like these, from our friends at Xerces.org:pollinator-friendly plants) in a comprehensive butterfly garden is a good idea.
It is important not to use pesticides in a butterfly garden, as the same chemicals that kill destructive insects will also kill butterflies and their larva.
Our thanks to the Spruce, from whom we sourced this info: https://www.thespruce.com/milkweed-plants-monarch-butterfly-host-2132954