The best time to plant a garden in Nebraska depends on the specific location within the state and the type of plants you want to grow. Nebraska is generally in USDA Hardiness Zones 4 and 5, with the southern part of the state being warmer. Key planting timelines for Nebraska are:

• Early Planting: Some cool-weather crops like radishes, asparagus, collards, onions, peas, and turnips can be planted as early as mid-March (around March 15), especially in the warmer southern part of Nebraska. Potatoes can also be planted in mid-March.
• April Planting: The critical date to keep in mind is around April 15. This is when you should ensure that the danger of frost has passed, with nighttime temperatures above 50°F and soil temperatures above 60°F for planting hardier vegetables and annual flowers. Broccoli, cauliflower, lettuce, and carrots can be planted in early April but may taste bitter if the weather is too cold.
• May Planting: By May 1, you can start planting heat-loving crops like cantaloupe, cucumber, pumpkin, and summer squash. Mid-May is recommended for eggplant, muskmelons, okra, peppers, sweet corn, sweet potatoes, and tomatoes. The last frost in many parts of Nebraska usually happens around early to mid-May (between May 3 and May 25 depending on the city).
• Summer Planting: Warm-season crops like beans, squash, corn, and cucumbers can be planted in early June.
• Fall Planting: Vegetables such as cauliflower, broccoli, and cabbage can be planted later in the growing season to flourish during cooler autumn days and can survive light frost.

Generally, the frost-free growing season in Nebraska starts from late April to early May and ends around late September to early October depending on the exact location. So, for most of Nebraska, starting any garden after mid-April is safest for most plants, while early spring plantings of hardy crops can begin as early as mid-March, and warm-season crops after mid-May.