Finding common milkweed food can be difficult in September because the plants become "crusty" in late August. By about Sept. 15, many leaves are dying. So, how can you find food for your monarch caterpillars?
If you cut off the top third of milkweed plants in mid- to late-July, the top will regrow and be much more tender in September.
To find food in September, look for milkweed areas--like a prairie--that have been mowed sometime during the summer. Milkweed plants there will re-sprout from the roots. These plants will be short and tender. But look carefully, because these small plants may not be obvious.
For example, county roads outside Madison have their shoulders mowed in mid-summer. It's easy to drive along until you spot a big clump of milkweed where the grass is shorter (because it was mowed). Look for an area with houses or woods around, not in the middle of agricultural fields (which probably have pesticide residues). You want to find a plant that is firm, not droopy, and not too old, hard, or crusty. Avoid plants covered with insects like ants, milkweed bugs, or aphids. But if you have to select plants with insects, you can wash them off with a hose.
To collect lots of milkweed, take scissors and a plastic bin. Put empty 32 oz deli containers, or old yogurt containers, in the bottom of the bin. Put an inch or two of water in each container. The large plastic bin keeps the harvested plants upright, while the plastic bin contains any sloshing water.
You could start feeding your caterpillars on another species of milkweed, such as swamp milkweed or butterfly weed. These also die out in September. Usually, I don't recommend these species of milkweed, to keep methods standard, and because the large leaves of common milkweed are easier to work with. However, if you do feed caterpillars with another species, keep in mind that they don't like to change from one species of food to another.