Kale is a popular, nutrient-dense leafy green known for its tolerance of cool temperatures, making it a favorite among gardeners seeking an early harvest. Starting kale seeds indoors is a common horticultural practice that allows the plant to gain a significant head start on the growing season. This method provides the seedlings with a controlled environment to develop strong root systems before the outdoor conditions are suitable for their permanent placement. Successfully moving these young plants from the sheltered indoors to the garden requires careful attention to both the seedling’s internal readiness and the external environment’s timing.
Assessing When Kale Seedlings Are Ready
The internal readiness of a kale seedling for transplanting is judged by its physical development. Seedlings generally reach an appropriate stage for moving outdoors when they display four to six fully developed true leaves. These true leaves appear after the initial cotyledon leaves and display the characteristic shape of the mature kale plant.
This developmental stage is typically reached when the seedling is approximately four to six weeks old. At this point, the plant has established a robust root system capable of handling the disruption of transplanting. A seedling that is too small may not survive the shock, while one that is root-bound will struggle to establish itself in the garden soil.
Determining Optimal Outdoor Timing
The correct transplanting date is determined by the last expected frost date for your region. Kale is a cool-season crop and seedlings can be set out four to six weeks prior to the last frost. Young kale plants tolerate air temperatures down to 25°F without serious damage.
While air temperature is important, soil temperature is a more precise measure of readiness for the root system. Kale can grow when the ground temperature is above 45°F, but it thrives best when the soil is between 60°F and 70°F. Using a soil thermometer to confirm temperatures consistently above 45°F ensures the plants transition smoothly and begin active growth immediately. Transplanting early allows the kale to mature before the intense heat of summer, which can cause the leaves to become bitter.
The Essential Hardening-Off Process
Before seedlings leave the protection of the indoors, they must undergo hardening off to prepare them for outdoor elements. Hardening off is a gradual transition that acclimates the plant to direct sunlight, wind, and fluctuating temperatures over seven to ten days. Skipping this step often results in transplant shock, as the tender foliage is instantly damaged by ultraviolet light and wind.
The process begins by placing the kale seedlings in a sheltered, shady outdoor location for one to two hours on the first day, preferably when the temperature is above 45°F. Each subsequent day, incrementally increase the duration outdoors and gradually expose them to more direct morning sunlight. By the end of the process, the plants should tolerate a full day outside.
Seedlings should still be brought indoors or covered if overnight temperatures are expected to drop significantly below freezing. Monitor soil moisture closely, as increased exposure to wind causes the soil in the containers to dry out faster.
Step-by-Step Transplanting and Immediate Care
Once the seedlings have been properly hardened off, transplanting should be done on a cloudy day or late in the afternoon to minimize sun stress. Prior to removal, thoroughly water the seedlings to ensure the root ball is saturated and less likely to fall apart, which helps minimize root disturbance. Dig a hole in the prepared garden bed that is twice as wide as the container and approximately the same depth.
Carefully remove the seedling from its pot, handling it by the root ball or a leaf, not the delicate stem. Set the plant into the hole so the top of the root ball is level with the surrounding soil, and avoid burying the stem too deeply. Kale should be spaced 12 to 18 inches apart to allow for mature growth and adequate air circulation.
Backfill the hole with soil, gently firming it down around the plant base to eliminate any large air pockets. Immediately after planting, water the newly transplanted kale thoroughly to settle the soil and help the roots establish contact with the earth. In the first few days, a temporary shade cover or row cover can be used to protect the young plants from intense sun or pests while they adjust.