Radishes are one of the most rewarding crops because of their low maintenance and quick harvest. As the fastest-growing root vegetables, they are prolific, making them perfect for greenhouse gardening. They’re also great for beginner gardeners looking to put their raised beds to work and take advantage of the nice weather.

Not only that, but with a greenhouse, gardeners can extend the growing season into early spring or even late fall, allowing for fresh radishes almost year-round. Here’s everything you need to know about growing radishes in a greenhouse, from selecting the best radish varieties to harvesting at the perfect time.

1. Varieties of Radishes

Spring radishes, like Easter Egg and French Breakfast, are ideal for early spring planting as they mature quickly and thrive in cooler conditions. These are the traditional red radishes commonly found in grocery stores. 

Another variety to consider is winter radishes, such as Daikon and White Icicle, which thrive in cooler weather. Although they grow more slowly than spring varieties, they produce much larger and more robust roots, making them ready for harvest in late fall or early winter, when the growing season winds down.

2. Growing Conditions

Light and Sunlight: Radishes need at least 6 to 8 hours of sunlight daily to grow at their regular speed and keep in mind too much shade results in slower growth and smaller crops. Full sun encourages strong growth and better flavor. 

Water: Consistent soil moisture is key for healthy radish development, but over-watering can lead to root rot, so avoid soggy soil. As a general rule, about one inch of water per week is sufficient to keep radishes thriving.

Temperature: Radishes prefer cooler temperatures, typically between 55°F and 75°F. They are best sown in early spring, once the frost risk has passed, but can grow well into fall if temperatures stay within this range. If temperatures exceed 75°F, radishes may bolt, resulting in flowers and bitter-tasting roots.

Soil: Radishes prefer soil with a slightly acidic to neutral pH, typically between 6.0 and 7.0. Avoid heavy, compacted soils, as they can restrict root growth and lead to small, misshapen radishes. Instead, opt for loose, fertile soil that allows the roots to grow freely and develop properly.

Fertilizer: Radishes are not heavy feeders, but a light application of fertilizer can help them grow strong and healthy. Avoid over-fertilizing, as too much nitrogen can cause excessive leaf growth and small, underdeveloped roots. Instead, use a balanced fertilizer or one that is lower in nitrogen to ensure the radishes focus on root development.

3. Germinating, Seedling Care, and Transplanting

Germinating Radish Seeds: Radish seeds are easy to sow and usually germinate quickly, typically within 3 to 7 days. Plant them about ½ inch deep in well-prepared soil and space the seeds about 1 inch apart. Larger varieties, like Daikon radishes, need more room to grow properly.

Seedling Care: As the seeds sprout, it’s important to thin the seedlings to ensure they have enough space to grow. For small-root varieties, aim for 1 inch between each seedling, while larger varieties need more space for their roots to fully develop.

Transplanting: Radishes generally don’t need frequent transplanting, but in a greenhouse, you can start them in trays or large containers before moving them to larger spaces. Once seedlings have developed their true leaves, they can be transplanted or thinned into the soil.

4. Common Obstacles and Issues

Crowding and Poor Spacing: When radishes aren’t thinned properly, overcrowding can easily occur, which leads to smaller, misshapen roots as the plants compete for space and resources. Without enough room to grow, their roots can't reach their full potential, causing stunted development, making proper spacing a crucial part of healthy radish growth.

Bolting in Early Warm Weather: Radishes are particularly sensitive to temperatures above 75°F, so if a greenhouse gets too warm, the radishes will bolt, meaning they start producing flower stalks instead of developing edible roots, resulting in bitter, undesirable radishes, negatively impacting the overall crop.

Pest Infestations: Pests like cabbage maggots, flea beetles, and root maggots are common, especially in cooler conditions, and they can wreak havoc on radish plants by damaging them, stunting their growth, and reducing yields. To help combat this, planting companion plants like marigolds or garlic can naturally deter insects and protect the radish crop from these pests.

Soil and Fertilizer Issues: If the soil is too wet or heavy, it can lead to root rot, which hinders growth and prevents healthy radishes from forming. Additionally, using too much nitrogen can encourage excessive foliage growth, but at the cost of small, underdeveloped roots, ultimately impacting the overall quality and size of the harvest.

5. Harvesting

Radishes should be harvested when the roots have reached about 1 inch in diameter and are firm to the touch, although the exact timing can vary depending on the variety. The best time to harvest is when the radishes are vibrant in color and feel solid, signaling they’re ready for picking. 

To harvest, gently pull them from the soil, taking care not to damage the roots. Typically, radishes will be ready for harvest 3 to 6 weeks after sowing, with smaller varieties maturing faster and larger ones taking a bit longer. It’s crucial to harvest them at the right time, as leaving them in the soil too long can cause them to form seed pods, which results in bitterness and a less desirable flavor.

Final Thoughts

Radishes are a crop that’s hard to mess up; they’re one of the easiest and fastest vegetables to grow, but there’s still room for error if they’re not cared for properly. If this is true for radishes in regular raised garden beds, then they must be even easier and more prolific in greenhouses. It’s up to you to take advantage of a small spot in the greenhouse and let them thrive.