I realize ivy is considered an invasive plant, however, we like it and are using it for erosion control. We recently planted cuttings with the help of root hormone powder. My question: is this an okay time to have planted this so that it can thrive?
You are correct in saying that English ivy is considered an invasive plant. It grows prolifically in the Pacific Northwest and has significant negative impacts on our native flora. It has also been declared a noxious weed by the Oregon Department of Agriculture, which means that Oregon prohibits the import, transport, propagation, or sale of English ivy.
Fortunately, there are many other native plant options for erosion control, such as low Oregon grape (Mahonia nervosa) or the compact variety of tall Oregon grape (Mahonia aquifolium 'Compacta'). You can also reference this handout from West Multnomah Soil & Water Conservation District for other native plant options (organized by the amount of sun and moisture available): Pacific Northwest Native Plants for Erosion Control.
