Brussels sprouts are a cool-weather crop requiring a long growing season to produce firm, flavorful sprouts. Successful cultivation relies heavily on timing the move from their indoor start to the garden bed. Proper timing is important because the plants must mature during cool fall weather, which enhances flavor by converting starches into sugars. This ensures the plants reach sufficient size before summer heat slows growth and that harvest occurs after the first fall frosts.
Optimal Timing Factors
The decision of when to transplant Brussels sprouts is governed by both the readiness of the seedling and the readiness of the environment. Seedlings are typically ready to move outdoors when they are four to six weeks old and have developed four to six true leaves. This size indicates the plant has enough photosynthetic capacity to handle the transition.
Environmental conditions are paramount, as these plants thrive in cooler temperatures, ideally between 60°F and 65°F. Daytime temperatures consistently above 70°F can cause stress or bolting, negatively impacting sprout quality. For a fall harvest, transplants are generally set out 90 to 110 days before the first anticipated fall frost.
The soil temperature should be a minimum of 50°F for planting. Gardeners in northern regions may transplant in early spring for a summer harvest, but this is only successful in areas with consistently cool summers. In most regions, planting in early to mid-summer for a fall or winter harvest is recommended to avoid peak summer heat.
Preparing Seedlings for Outdoors
Before planting, indoor-started Brussels sprouts must undergo a process called “hardening off” to acclimate them to the harsh outdoor environment. Seedlings grown indoors are accustomed to stable temperatures and filtered light, making them susceptible to shock if moved suddenly. Hardening off toughens the plant tissues and prevents sun scald, wind damage, and a check in growth.
The process typically spans a period of 7 to 14 days, gradually exposing the young plants to the elements. Begin by placing the seedlings in a sheltered, shady location outdoors for two to three hours on the first day. This protected spot allows them to adjust to the temperature change and natural air movement.
Each subsequent day, increase the time spent outdoors by an hour or two, moving the plants into slightly more direct sunlight. After several days, the seedlings can be exposed to morning sun. By the end of the two-week period, the plants should tolerate a full 24-hour cycle outside, including overnight temperatures above 40°F, indicating they are ready for permanent planting.
The Physical Transplantation Process
Once the timing is correct and the seedlings are properly hardened off, prepare the planting site. Brussels sprouts require a full-sun location with fertile, well-drained soil that has been amended with aged compost or organic matter. This addition helps meet the plant’s high nutritional needs and ensures the soil retains moisture while preventing waterlogging.
When transplanting, gently handle the seedlings to avoid disturbing the root ball, as damaged roots can lead to transplant shock. Set the seedlings slightly deeper than they were growing in their original containers, burying a portion of the stem up to the lowest set of leaves. Planting deeper helps anchor the tall plants and encourages stronger root development.
Proper spacing is necessary to allow for adequate air circulation and light penetration, which helps deter fungal diseases and promotes robust sprout formation. Transplants should be spaced about 18 to 24 inches apart in the row. Immediately after planting, water the seedlings thoroughly to settle the soil and eliminate air pockets. If the weather is sunny or warm, providing temporary shade for the first day or two can further mitigate transplant shock.