One of the surest ways to see fall-migrating Monarch butterflies is to plant flowers that attract them. Monarchs will drop from the sky for the nectar they need for energy during fall migrations.
(For your convenience, you can follow links on the various plants mentioned here to check for availability and price.)
Many Lantanas (Lantana spp.) are still blooming. Last year I had several Monarchs stop in late October in my North Carolina, USA, garden to sip the nectar from ‘Miss Huff’ Lantana (Lantana camara ‘Miss Huff’)
Goldenrod (Solidago spp.) is a wonderful fall blooming perennial that attracts Monarchs and comes in many different varieties.
Ironweed (Vernonia spp.) always attracts Monarchs.
Other great nectar flowers to plant for fall-migrating Monarchs include these:
Purple Coneflower (Echinacea purpurea)
Autumn Joy Stonecrop (Sedum ‘Herbstfreude’)
Dense Blazingstar (Liatris spicata)
Showy Goldenrod (Solidago speciosa)
Anise Hyssop (Agastache foeniculum)
Joe-Pye Weed (Eupatorium fistulosum)
Rose Verbena (Verbena canadensis)