Written by Dale Richardson - Updated: June 23, 2023
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Carrots are not usually thought of as a greenhouse crop. However, these delicious root vegetables can be grown and harvested for a good part of the year inside a greenhouse. Let's look at how to grow carrots in a greenhouse.
Growing carrots in a greenhouse can either allow you to plant earlier in the spring, or extend your harvest into the late fall and early winter. It is important to provide adequate water and sunlight, maintain appropriate soil and air temperature, and protect the carrots from pests.
Growing carrots in a greenhouse is similar to growing them in a field, but there are a few special considerations that need careful management. This will hopefully provide you with enough insight to see how you can best manage your greenhouse to have a successful and abundant carrot crop.
When growing carrots in a greenhouse, they can be planted directly in the ground, in planters or pots, carrot grow bags , or in raised beds. Which method you prefer is up to you and should match your goals and management plan.
Carrots do not do well in heavy, clay soils as it impedes root growth. Carrots prefer loam, sand, or sandy loam. Add a good mix of compost and work the soil so it is not compacted to match the depth of your carrot variety. Most carrots prefer about 12" of soil to properly grow. A good soil PH is neutral to slightly acidic between 6 and 7.
If you are a coffee drinker, adding coffee grounds to your carrot patch helps create a nice soil tilth, adds a small amount of nitrogen, and adds the needed bit of acidity to the soil.
Since greenhouse carrots do not receive any rainfall, you will have to regularly water them. Carrots can be over 90% water, so they need consistent water to grow nice, healthy roots for you to eat. The soil should stay damp but not soggy or the carrots might rot out. It is especially important to keep the seeds watered during germination, otherwise, the seeds will die if they keep drying out.
Carrots require 2.5cm (1inch) of water each week to properly grow. This is equivalent to about 2,300L 93 sq m (623 gallons of water per 1,000 sq ft). A more practical way to know if you are adding adequate water is to a rain gauge amongst your carrots. As you water the vegetables, the rain gauge will fill and you will be able to see how much water they have received. It is best to spread the water throughout the week rather than give them a deluge on a single day so the plants have consistent moisture to develop.
READ NEXT: 8 Tips for winterizing your greenhouse.
Insects can be a problem in any garden. A greenhouse not only creates a perfect environment for carrots to thrive, but it creates the ideal breeding ground for many bugs. Once an infestation starts, it is very difficult to eradicate.
Carrot flies are particularly dangerous. Carrot flies are attracted to the scent of carrot tops and then lay their eggs in the surrounding soil. The hatched larvae will quickly eat through a crop, turning a beautiful harvest into a maggot-infested mess. There are many natural methods to resist the carrot fly, such as companion planting, crop rotation, and avoiding thinning.
If you plan on growing carrots in a greenhouse, it worthwhile to check if carrot flies or any other bug is a major concern in your area, and have a plan of attack ready before the invasion begins.
One of the biggest questions about growing any vegetable in a greenhouse is whether the greenhouse should be heated or unheated. This mostly depends on the temperatures you are facing in your area, and why you want to grow carrots in your greenhouse (see below).
A heated greenhouse has the obvious advantage that it will keep your carrots warm no matter what the weather is doing outside. It gives you a lot more flexibility of when you can plant, and you might be able to grow all year long. The downside is the heating bill and the environmental impact. You will have to decide if the cost and fuel consumption are worth the off-season vegetables.
Unheated greenhouses create a warm environment using nature's own phenomenons. However, the power of the sun is limited in many regions over winter. Depending on where you live, you might not be able to grow carrots 12 months of the year. But an unheated greenhouse might just give you the extra warmth you need to grow bountiful carrots in every season.
Whether your greenhouse is heated or unheated, be wary of growing your carrots in the greenhouse over summer. Carrots are a cool-season crop and quickly overheat. If your carrots are inside over summer, make sure your greenhouse has adequate ventilation.
One benefit of growing carrots in a greenhouse is that you can improve your harvest and increase your yield. Because a greenhouse is a controlled environment, you can manage more minutely each aspect of the carrots growing, such as soil amendments, water level, and of course temperature. In essence, you can provide the perfect growing conditions for your carrots. A greenhouse also extends your growing season, which can greatly increase your yield for the year.
Of course, this type of gardening requires careful planning and attention to detail. It also is more labour intensive: if you are feeling particularly lazy on a rainy afternoon, you will still have to go and water your carrots in the greenhouse.
This is by far the most common use of a greenhouse. Even in many temperate climates, it is too labour intensive and too risky to grow carrots 12 months of the year. A greenhouse would allow you to sow carrots in late winter for an early harvest. The important thing to know when is the best time to plant your greenhouse in your area. A major consideration is light, since the winter sun is usually too weak, and the days are too short to provide successful germination.
In most northern climates, a rule of thumb is to start planting in the middle of February to jump-start your carrots, but it is a good idea to ask a local garden centre for their recommendation.
Image by Axel Kristinsson
Depending on where you live (or if your greenhouse is heated), you can grow carrots all year long. You can use the greenhouse as a cold storage by keeping your carrots in the ground all winter. Even in colder climates, the combination of the greenhouse and a think mulch of straw can keep the ground from freezing, so you can harvest your carrots as needed throughout the winter.
Alternatively, if your winters are mild enough, you can plant carrots in late fall and harvest them over Christmas and into the new year. Remember, sunlight is key. It is difficult to germinate your carrots in the cold months where there is limited sunlight, so it is best to have them already up and growing by the time the days start getting shorter. Besides the sunlight, carrots do best with a soil temperature of 7°C to 30°C (45°F-86°F). They can survive colder, but it is best to keep them in this range if you want your carrots to continue growing.
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