This video from the USA says about itself:
Growing Milkweed for Monarch Butterflies
6 December 2013
Monarch butterflies have suffered a severe population decline and you can help provide habitat for them during their migration by growing milkweed. There are more than 40 varieties, one perfect for your climate. Christy Wilhelmi from Gardenerd shows you how and why we need to grow milkweed. Make a plan to grow it this spring!
From the Cornell Lab of Ornithology in the USA:
If you’re looking for a great native addition to your yard, take a look at milkweed and this helpful article from our YardMap project. Milkweed is crucial for monarch butterflies and for many native pollinators. With more than 100 species native to North America, it’s a vibrant but vanishing plant that will look great in your yard. Our article has info on which species grow near you, where to find seeds, and more.
The number of Monarchs migrating from the U.S. has dropped by 90 percent in the past 20 years: here.