COLUMBIA, Mo. (AP) – Volunteers in Columbia, Missouri, have been working to create a sanctuary for millions of monarch butterflies that pass through the state as they migrate to and from Mexico.

The Columbia Missourian reports that efforts to plant milkweed and other native flowers on park land were boosted earlier this month when Columbia became the 16th city in the state to join the National Wildlife Federation’s Mayors’ Monarch Pledge. On a recent Saturday, volunteers spent four hours cutting bush honeysuckle with hand saws, piling up the debris and applying herbicide to kill an invasive species on the five-acre field.

Native seed, including milkweed and up to 40 different native flowers, can be planted once the field has been cleared of unwanted plants that out-compete native species, perhaps as soon as 2019.