When is the best time to plant summer bulbs? While some horticulturists say you can plant them in September, most gardeners plant them in early spring.
Plant bulbs as soon as the ground can be worked. Given how delicate bulb flowers look, you may think they need a lot of babying. Nope. Summer bulbs, such as lilies and gladiolus, can be planted as soon as the ground is thawed and reasonably dry—typically mid-April, according to a 1953 article in Minnesota Horticulturist. Gladiolus bulbs can be planted every two seeks from April to mid-June for a staggered harvest of beautiful blooms for bouquets.
Wait to plant dahlias until mid-May. Dahlias, which tend to bloom later in the season, prefer a later plant date. In 1909, D.W.C. Ruff of St. Paul recommended waiting until May 15 to plant dahlais. “June 15th is not too late for best results,” he wrote. Dahlias are adaptable to many soil types, though they do not like to be overly wet. A sunny location will produce the biggest blooms, but they can handle a bit of shade. To battle cutworms, Mr. Ruff recommended “a little air-slaked lime or poised bran placed on the surface of the soil,” though today gardeners are more likely to use cardboard collars around the young plants and other deterrents.
Use organic fertilizers in spring. In 1975, long-time Minnesota Horticulturist columnist Fred Glasoe recommended against adding chemical fertilizers to dahlias in spring. “Too often the grains of these concentrated elements burn the hair roots and set back growth for many weeks,” he wrote. “Good organic humus that is light and can be tossed around with ease is what your dahlias will thrive on.”
Keep protection handy in case there is a late frost. Depending on the weather, lily tips may be emerging from the soil. If the weather is fine, you can brush aside the leaves or other winter cover you had on your plants, Jerry Brown of the North Star Lily Society wrote in 1976. But keep something close by that you can cover the emerging foliage with, if a sudden frost threatens. Leaves, a pail or a bucket will provide enough protection. If this is a recurring problem, consider where you have placed your lilies. A south-facing slope or warm micro-climate may cause the lilies to emerge too soon. A northern exposure might be safer, he noted, and suggested that lilies be covered with leaves or mulch after the ground freezes in the fall to keep them from coming out of dormancy too early.
Martagon lilies thrive in our cold climate! Martagon lilies with their down-facing flowers are slow to spread and reproduce, making them one of the most costly summer bulbs, but they are magnificent in bloom and they thrive in our cold climate, according to Frans Officer, an amateur martagon hybridizer who wrote about his favorite plant in Northern Gardener in 2010. Martagons take up to seven years from seed to flower. While some cultivars readily make new bulbs underground and grow into big, beautiful clumps, others don’t. If you want to grow martagons, they love dappled shade and humus-rich but well-draining soil. But they can take full-sun with adequate moisture. While a bit more work and money than other summer bulbs, martagons love the North and often will put on their best show after an extremely cold winter, Officer wrote.