The best month to plant butterfly bush is April or May in most parts of the country. Late spring gives your plant the full growing season to set roots. Skip fall planting in cold zones if you want strong survival rates.
I tested both timing options back in 2018 in our Zone 6 yard. I put one shrub in the ground in late April. I planted a second one in mid-October that same year. The spring plant came back strong the next spring and bloomed on time. The fall plant rotted in the wet winter soil and never sent up a leaf the next spring.
Spring works best for one big reason. Your shrub needs the full growing season to grow a deep, strong root web. Roots that reach 18 to 24 inches deep by fall give the plant a fighting chance through winter. A fall plant has just a few weeks to root before cold sets in, which spells trouble.
When you think about when to plant butterfly bush, frost dates set the start line. Wait until after your last spring frost date to plant. In Zone 5, that means mid-May or even early June. In Zone 6, late April works well in most years. In Zone 7 to 8, you can plant as early as late March or April when soil warms up.
Soil temperature matters more than calendar date on its own. Wait until soil hits 50°F (10°C) at a 6 inch depth before you plant. You can pick up a soil thermometer at any garden store for under 15 dollars. Stick it in the ground at midday for a true reading of your soil temp.
Spring planting butterfly bush also dodges the heat stress of summer. Plants that go in during June or July struggle in hot, dry weather. New transplants have small root webs that cannot keep up with water loss. Leaves wilt, growth stalls, and many plants die in their first month in the ground.
I found that the sweet spot in my Zone 6 yard runs from late April to mid-May. Soil temp hits 50°F right around April 20 in most years. Daytime air temps stay below 75°F (24°C) through May. Rain comes often enough to keep new roots moist without my help on most days.
If you must plant late in the season, give your shrub extra care from day one. Water deep twice a week for the first month after planting. Add a 3 inch mulch layer to hold soil moisture and cool the root zone. Skip fertilizer at planting time since it can burn fresh roots when they need them most.
Fall planting works only in warm zones (7 to 9) with mild winters and good drainage. September to early October gives the plant 6 to 8 weeks of root growth before dormant time. The plant goes into winter with a small but healthy root web. Avoid this method in clay soil or low spots where water collects.
Pick a cool, cloudy day for the actual butterfly bush planting time. Hot sun on transplant day stresses the plant from the start. Morning is better than afternoon if you must plant on a sunny day. Water the plant well in its pot the night before you plant it in the ground.
Dig a hole twice as wide as the root ball but the same depth as the pot. Skip the temptation to plant deeper than the soil line on the trunk. Buried crowns rot fast in wet soil. Set the plant so the top of the root ball sits flush with or just above the soil line in your bed.
Water deep right after planting to settle the soil around the roots. Use 5 gallons of water for a standard one-gallon plant. Keep up that deep watering pattern once or twice a week for the first month. Cut back to once a week through summer of year one to build a strong root web.
Read the full article: Butterfly Bush: Complete Growing Guide
