March is a significant transition point for gardeners seeking vibrant summer color. The bulbs, corms, and tubers planted now are classified as “tender” because they originate from tropical or subtropical climates and cannot tolerate freezing. They must only be placed in the ground after the danger of the last hard frost has passed. This early spring planting establishes robust root systems and healthy foliage, ensuring a spectacular display of flowers that lasts from mid-summer until the first autumn frost.
Tender Bulbs for Summer Color
Dahlias are beloved for their diverse forms and sizes, ranging from small, globe-shaped pom-poms to massive, dinner-plate blooms. As tubers, they are extremely sensitive to cold, so planting directly into the garden soil is best done when the ground temperature consistently reaches 60°F. For the largest summer display, many enthusiasts choose to begin their Dahlia tubers indoors in pots about four to six weeks before the anticipated outdoor planting date, which effectively extends the growing season.
Gladiolus, which grow from corms, offer striking vertical interest with tall, sword-like foliage and densely packed flower spikes. These can be planted sequentially every two weeks starting from the last frost date until early summer to ensure a continuous succession of blooms throughout the season. Their relatively shallow planting depth makes them susceptible to damage from late cold snaps, so careful monitoring of weather forecasts is important after the initial planting.
Tuberous Begonias and Caladiums represent two groups of tender plants that thrive in partial shade, unlike many sun-loving summer bulbs. Begonias are often started indoors in March because their slow initial growth rate requires a head start to produce their signature large, waxy flowers by mid-summer. These plants perform best when the soil is kept evenly moist and the ambient temperature remains warm.
Caladiums are known for their colorful, patterned foliage rather than their flowers, providing texture and brightness in shady garden spots. These heat-lovers should only be moved outside once both the air and soil temperatures are reliably warm, remaining above 65°F. Waiting until the soil is thoroughly warm prevents the tubers from sitting dormant or potentially rotting.
Canna Lilies are large, dramatic tropical tubers prized for their bold, banana-like foliage and fiery, often red, orange, or yellow blooms. Due to their long growing requirement, Canna Lilies benefit from being started indoors in containers in March in warmer climates or early April in cooler regions. Starting them early ensures they reach their full height and flowering potential quickly once transplanted into the garden.
Understanding Soil and Site Conditions
Success with tender bulbs begins with preparing the correct environment for their growth. The most important factor is adequate soil drainage, as standing water rapidly leads to suffocation, rot, and fungal diseases. To assess current site drainage, dig a hole approximately one foot deep, fill it with water, and observe how quickly the water dissipates. If the water remains in the hole for more than a few hours, the site requires modification before planting can occur.
Heavy clay soils, common in many regions, benefit from the thorough incorporation of organic material such as well-aged compost or peat moss, which improves both aeration and structure. Adding coarse grit or sharp sand helps to break up dense soil particles, facilitating the movement of water away from the sensitive bulb structure.
Light exposure must align with the specific needs of the chosen variety to ensure flowering and healthy growth. Most summer bulbs, including Dahlias and Gladiolus, require a minimum of six to eight hours of direct sunlight daily for robust growth. Conversely, plants like tuberous Begonias and Caladiums will suffer from leaf scorch in full sun and perform best when situated in an area that receives bright, filtered light or morning sun followed by afternoon shade.
The planting site should be protected from strong winds, which can damage the tall, heavy stems of plants like Canna Lilies and Gladiolus. While amending the soil, incorporating a balanced, slow-release fertilizer into the planting area gives the bulbs the nutrients they need as they begin their active growing phase. Proper site preparation ensures that the bulbs immediately begin rooting and storing energy for their summer display.
The Mechanics of Planting
Once the planting site is prepared and the soil is warm, the physical placement of the bulb, corm, or tuber requires attention. A general guideline for planting depth is to cover the bulb with soil two to three times its vertical height; consequently, a two-inch tall corm should be buried approximately four to six inches deep. Planting too shallowly can cause the plant to become unstable and fall over once it reaches its mature height, while planting too deeply can significantly delay or entirely prevent blooming.
Proper orientation is a factor, particularly with irregularly shaped tubers and corms, which must be correctly positioned for the shoot to emerge. The pointed end or the location where the previous year’s stem was attached, often called the “eye,” should face directly upward toward the soil surface, as this is the point where the new shoot will emerge. Tuberous Begonias are often concave, and this hollow side should face up to collect moisture and protect the emerging growth bud.
Spacing is determined by the mature size of the specific plant, which ensures adequate air circulation and room for growth. Large Canna Lilies may require two to three feet between plants to accommodate their broad foliage, while corms such as Gladiolus can be spaced closer, approximately four to six inches apart. Immediately after planting, a thorough initial watering settles the soil around the bulb and initiates root development.
If planting is undertaken early in March and a late frost is predicted, covering the area with straw or a temporary horticultural row cover provides necessary insulation. This protects vulnerable new shoots or shallowly placed bulbs from freezing temperatures. Consistent moisture during establishment encourages strong root formation for a spectacular summer bloom.