There are a number of ways to set up milkweed in your rearing house while you raise Monarch caterpillars.
The key is to make sure the caterpillars cannot fall into water….. they will drown. It is also critical to have a good rearing cage so the caterpillars do not wander off and die.
Here is a great tip sheet from Monarch Joint Venture on Raising and Releasing Monarchs.
Poke holes in a plastic container lid. Put water in the container, replace lid. Then stand milkweed leaves up in the holes. Milkweed leaves stay fresh and frass (poop) falls down. This is our favorite method.