What plant were the Israelites to dip in the blood to brush it on their doorframes?

Answer: Hyssop

Moses told the elders to dip a cluster of hyssop in the blood in the basin and brush it on the top and sides of the doorframe.

“Take a cluster of hyssop, dip it into the blood in the basin, and brush the blood on the top and sides of the doorframe. None of you shall go out the door of his house until morning.”— Exodus 12:22 (BSB)