You can’t have monarch butterflies without specific milkweed plants (Asclepias spp. If you plant bad seeds in spring, by the time you realize it, it may be to late to replant for that season. In spring, pull back the mulch to give space to the new growth. Like all wildflowers, milkweed should be planted on bare soil. Swamp Milkweed blooms for approximately 1 month, making it a longer-blooming perennial. Select The Right Site Best flowering usually occurs in full sun except for a few species such as Asclepias purpurascens (Purple Milkweed), which prefers partial shade. If anything we sell fails to grow in your garden, we will replace or refund your order. Good luck!