Gladiolus, often called the “sword lily” due to its tall, dramatic flower spikes, is a popular choice for cutting gardens. These vibrant summer bloomers grow from subterranean storage organs known as corms, which are frequently mistaken for true bulbs. Successful cultivation in a cold climate like Wisconsin depends on precise timing and preparation to ensure a spectacular display throughout the warm months.
Timing the Planting
The correct planting time for gladiolus corms depends primarily on soil temperature. Planting must wait until the risk of the last hard frost has passed, as corms are susceptible to rot in cold, saturated ground. The soil temperature must consistently reach a minimum of 55 to 60 degrees Fahrenheit before planting safely commences. This requirement places the ideal planting window in Wisconsin typically from mid-May through early June.
Planting too early increases the chance of fungal diseases and corm deterioration. For a sustained harvest, use staggered planting by setting a new batch of corms every two weeks until early July. This technique extends the blooming season from late summer into fall.
Preparing the Planting Site
The chosen location requires specific conditions to support the gladiolus’ tall growth habit. A planting site must receive full sunlight, meaning at least six to eight hours of direct sun exposure daily. Adequate light is necessary for the plant to produce a strong stem and abundant flowers.
The soil composition is equally significant, as corms demand excellent drainage to prevent water accumulation. Heavy Wisconsin clay soil can be fatal, causing corms to rot quickly. To improve drainage and aeration, amend the soil by incorporating organic materials like well-aged compost, peat moss, or aged manure before planting. This preparation ensures the site is hospitable by allowing excess water to drain away effectively.
Planting Depth and Technique
Placing the corm into the prepared ground is critical for proper establishment and stem stability. Large corms should be positioned four to six inches deep; smaller corms, or cormels, can be planted slightly shallower. This depth provides necessary anchorage for the mature plant, which can grow tall and become top-heavy when in full bloom.
When planting, the pointed end containing the growing point must face upward toward the soil surface. Spacing the corms approximately six inches apart prevents overcrowding, ensuring each plant has sufficient light, air circulation, and nutrients. After covering the corms with soil, an initial thorough watering helps settle the earth and initiates growth.
Winterizing Gladiolus Corms
Gladiolus corms are not reliably cold-hardy in Wisconsin’s climate, which primarily falls within USDA Hardiness Zones 3 to 5. Consequently, they must be lifted annually to ensure their survival, as they cannot be left in the ground to overwinter. Lifting should occur after the foliage naturally begins to yellow and die back, but before the first hard frost, which typically arrives in October.
The foliage should be cut back to within one or two inches of the corm before carefully digging up the plant using a garden fork. Once lifted, the corms need curing to dry and toughen their outer layers, preventing rot during storage. Curing involves placing the corms in a warm, dry, and well-ventilated area for one to three weeks at 70 to 80 degrees Fahrenheit.
After curing, the old, withered mother corm at the base should be removed and discarded, leaving only the firm, new corm and any attached cormels. The cleaned corms are then ready for storage in a medium that allows air circulation, such as dry peat moss, vermiculite, or nylon mesh bags. Stored corms must be kept in a cool, dark location, ideally between 40 and 50 degrees Fahrenheit, until the planting window reopens the following spring.