March is a transitional period in gardening, dependent on your USDA Hardiness Zone and the average date of your last expected frost. This month requires a dual strategy: starting long-season flowers indoors and planting cold-tolerant varieties directly into the awakening soil. Understanding which flowers thrive early ensures a beautiful summer display.
Starting Seeds Indoors
March is the optimal time to sow seeds for flowers that require a lengthy period to germinate or those classified as tender annuals, which cannot handle the cold spring soil. These varieties need a sheltered environment to develop into robust seedlings ready for transplanting after the danger of frost has passed, usually in May or June. Starting early ensures a long and abundant flowering season later in the year, maximizing the plant’s full potential.
Flowers such as petunias, snapdragons (Antirrhinum), and cosmos are excellent candidates for indoor sowing this month. Petunias, in particular, need an early start due to their slow initial growth, and they will reward you with continuous blooms throughout the summer. Tender perennials, like verbena, also benefit from a March start to ensure they flower in their first year.
Successful indoor seed starting relies on three main factors: light, temperature, and moisture. Place seed trays in a warm spot, ideally maintaining 65°F to 75°F for optimal germination of half-hardy annuals. Since natural sunlight is insufficient in March, supplementing with a dedicated grow light is necessary to prevent seedlings from becoming weak and leggy. Keep the seed-starting mix consistently moist, but avoid saturation, which can lead to fungal issues like damping-off disease.
Hardy Varieties for Outdoor Planting
March provides an opportunity to plant cold-tolerant flowers directly outdoors. These hardy varieties are generally frost-tolerant or require a period of cool soil and temperatures to trigger germination, a process known as vernalization. This planting is distinct from indoor starting because the seeds or plants are placed directly into the garden bed.
Direct sowing of seeds in March is perfect for hardy annuals like Californian poppies, cornflowers (Centaurea cyanus), and calendula. These plants dislike having their roots disturbed, making direct seeding the preferred method for establishing them. Sweet peas are another good choice for early outdoor planting, as the cool soil helps them develop strong root systems before the summer heat arrives.
Beyond seeds, you can also introduce certain established plants into the garden this month. Nursery starts of pansies and violas are specifically bred for cool weather tolerance and can instantly provide color to beds and containers. March is also the final opportunity to plant dormant bare-root perennials, such as roses, while they are still in their resting phase. Ensure that any plant material going into the ground is rated for the cold conditions still likely to occur in early spring.
Preparing the Planting Bed and Managing Frost Risk
The success of any early outdoor planting hinges on the condition of the soil and your strategy for managing unpredictable spring weather. Before planting, check the soil’s workability, ensuring it is not overly saturated with winter moisture. A simple test involves taking a handful of soil and squeezing it; if it crumbles easily, it is ready, but if it forms a tight, muddy ball, it is too wet and should be left to dry before digging.
Knowing your region’s average last frost date is important, as this date provides the benchmark for when truly tender plants can safely move outside. Early plantings of hardy varieties can be shielded from unexpected late freezes by keeping protective covers ready. Material like a lightweight row cover or an old blanket can be draped over plants in the evening to trap residual warmth from the soil.
Watering the soil thoroughly before a predicted frost can also offer a measure of protection, as moist soil retains heat better than dry soil. A two-to-three-inch layer of organic mulch, such as shredded leaves or straw, applied around newly planted specimens helps insulate the ground and stabilize soil temperatures. These simple protective measures can make the difference between a successful early start and losing newly planted flowers to a sudden cold snap.