Pansies
Pansies may be the most common northeast frost tolerant flower. Pansies are available in many different colors such as orange, blue, black, purple, white, yellow or pink with some varieties having a ‘face’ or a black or brown center like a viola. Bloom sizes can be large up to 4.5 inches, medium up to 3.5 inches or Floribunda up to 2.5 inches.
Dusty Miller
Dusty Miller has silvery-white fuzzy leaves and produces a pale to bright yellow-colored bloom in the summer. It can be planted as early as March in zone 9 but will thrive in all zones according to the University of Florida IFAS Extension.
Wildflowers
Merrybells or Bellwort is a wildflower that has small yellow flowers and blooms from April through June. It requires a location with full to partial shade and well drained soil. Merrybells grow to a height of 18 inches.
Bare Root Roses
Bare root roses can be planted as soon as the ground has thawed and is workable. When selecting bare root roses in the early spring, choose plants that are still dormant or that have not begun to grow green shoots from the stem. Be sure and give your new rose a good mixture of organic material to help keep the soil well drained and rich in nutrients, with about one inch of mulch added after planting. If your new rose begins to present new green shoots and you are aware of a possible frost, protect the new growth with straw, burlap or a paper grocery bag until the sun clears the frost for the day.
Seeds
Some flower seeds such as snapdragon or marigold can be planted early and may not germinate until the length of the daylight is appropriate for germination, protecting it from early frosts.
Fall Frost Hardy
Many flowers are frost hardy in the fall after they have had a full growing season to strengthen their roots and stems such as petunia and chrysanthemum, but are generally too tender to tolerate frost in the spring. Understanding the frost-hardiness of each type of plant in your garden will help you determine when and how to protect your plants from frost damage.
USDA Northeast Zones
According to the USDA Plant Hardiness Zone Map, the northeast states include Connecticut, Delaware, District of Columbia, Illinois, Indiana, Kentucky, Maine, Maryland, Massachusetts, Michigan, New Hampshire, New Jersey, New York, Ohio, Pennsylvania, Rhode Island, Vermont, Virginia and West Virginia. Consult your local extension office for a complete list of flowers that can be planted outside in April in the northeast region of the United States.