Start seeds for Brussels Sprouts approximately 5 to 6 weeks before you want to transplant them into your garden. Plant the seeds 1/4-inch-deep in seed starting medium. You may find a heat mat helpful for germination, but after your seeds pop up, they would prefer not to have warm soil.
Prepare the garden 2 to 3 weeks before planting. Till the soil and mix in organic compost. Brussels sprouts do best in loose organic soil that retains moisture. They grow best in full sun but will tolerate partial shade.
Brussels sprouts like a pH level of 6.0 to 6.5. Soil temperature should be 70-80°F (21-26°C) for optimal growth. Brussels Sprouts are heavy feeders and will benefit from applications of boron, calcium and magnesium, particularly during the early stages of growth.
Transplant your seedlings into the garden when they are 4 to 6 weeks old and around 6 inches tall.
Remove the plants from the seed pots. Soak the root balls in a general plant fertilizer mixed with water before planting them in the garden. (We like fish emulsion for this step, but use your favorite) Follow the fertilizer manufacturer's guidelines to prepare the correct concentration of fertilizer solution.
Space the plants 24 to 30 inches (61 to 76 cm) apart. If the transplants are spindly or growing crooked, you can bury them in the ground up to the first set of leaves so the plants do not become top heavy.
Water the plants at the base after transplanting. Keep them well watered throughout the growing season to prevent the ground from drying out. Reduce the amount of water a couple of weeks before harvest; allow the ground to dry between watering.
Feed Brussels sprouts with a nitrogen fertilizer every few weeks. Stop fertilizing when you cut back on watering at the end of the growing season.
In addition, removing the growing tip of the plant about a month before the harvesting date will result in greater yields as the plant will divert energy from leaf growth into developing the Brussels sprouts.
Add organic compost to the top of the soil around the plants regularly. This feeds the plants and provides a barrier for weeds. Brussels sprouts have a very shallow root system, so avoid disturbing the soil around them.
Harvest your Brussels sprouts slowly, starting at the bottom of the plant. The sprouts mature from the bottom up and are at their sweetest when they are still small and tightly closed.
To encourage development of the upper sprouts, pinch out the growing tip of the Brussels sprouts plant in late summer. The Brussels sprouts will develop a sweeter flavor after a few light frosts.