Cauliflower is a cool-season crop challenging for home gardeners due to its narrow preferred temperature window. Starting plants indoors is necessary to maximize the short growing season and avoid summer heat, which prevents proper development. Precise transplant timing is the single most important factor, ensuring the formation of a tight, edible “curd.” If growth is stunted or shocked during this transition, the plant will fail to produce a head.
Determining Optimal Seedling Readiness
Seedlings signal readiness when they are between four and six weeks old, having developed four to six true leaves. The true leaves indicate the plant is photosynthesizing efficiently enough for the move, unlike the initial cotyledons. At this stage, the plant will generally be between three and six inches tall.
Transplanting too early means the small, underdeveloped root system is too fragile to handle the outdoor environment, leading to failure. Waiting too long causes the seedling to become root-bound in its small container, which is a more common issue. A stressed, root-bound cauliflower plant is prone to “buttoning,” the premature formation of a small, unusable head. The goal is to move the young plant while it is actively growing and before its root system is constricted.
Hardening Off Procedures
Before any seedling is placed into the garden soil, it must undergo “hardening off,” which acclimates the tender plant to the harsh outdoor elements. This transition gradually exposes the young cauliflower to direct sunlight, wind, and temperature fluctuations over seven to fourteen days. Skipping this step results in immediate transplant shock and failure, as the plant cannot instantly tolerate the change in environment.
The process begins by placing the seedlings outdoors in a shady, protected location for only a couple of hours each day. Over the subsequent days, the duration of their time outside is slowly increased, and they are moved into areas with incrementally more sun exposure. Sun is much more intense outdoors than through a window or under a grow light.
For the first week, the plants should be brought back inside every night, especially if temperatures are predicted to drop below 45°F. During this time, the amount of water given should also be slightly reduced to help them toughen up their cell structure. By the end of the hardening-off period, the cauliflower seedlings should be able to remain outside overnight and withstand several hours of direct, unfiltered sun.
The Final Transplant Timing and Technique
The ideal time for the final transplant is approximately one to two weeks before the last expected spring frost date. However, the most reliable indicator is the soil temperature, which must consistently reach between 55°F and 65°F for the plant to thrive. Planting into cold soil below 50°F can stunt growth and induce premature buttoning, even if the air temperature is suitable.
The permanent garden bed should be prepared with rich, well-draining soil that has a high organic matter content and a pH between 6.0 and 7.5. When planting, the seedling should be placed into the ground at the same depth it was growing in its original container. If the seedling has become slightly elongated or “leggy,” it can be planted slightly deeper to provide better stem support.
Each cauliflower plant requires significant space to develop the large, leafy canopy necessary to support a full-sized head. Space the seedlings 18 to 24 inches apart, allowing enough room for air circulation and growth. Immediately after planting, water the seedlings thoroughly to settle the soil around the root ball and minimize transplant shock. If temperatures are unexpectedly high, providing temporary shade for a few days will help the young plants establish themselves.