Key Takeaways -

• Avoid plants that outgrow limited space or overshadow others, such as giant trees and shrubs.
• Steer clear of invasive species that rapidly spread and compete for resources.
• Select crops that aren’t prone to pests and diseases to prevent a breeding ground for issues.
• Ensure plants have compatible growing conditions for a balanced environment conducive to healthy growth.

Growing plants in a greenhouse has many benefits, such as extending the growing season and protecting plants from harsh weather. However, not all plants are suitable for greenhouse environments.

Some can grow too large, attract pests, or compete for resources, making it harder for other plants to thrive. This article will highlight the plants you should avoid growing in a greenhouse and why. It will also offer practical tips on choosing better alternatives for a healthy and productive greenhouse.

Keep reading to learn how to make the most of your greenhouse by selecting the right plants and avoiding common mistakes. Making informed choices can help you maintain a balanced and successful gardening space.

#1 Giant Trees and Shrubs

Giant trees and shrubs may seem appealing for a greenhouse, but they often outgrow limited space, making them unsuitable for confined environments. Their rapid growth can overshadow smaller plants, blocking essential sunlight needed for healthy growth.

Additionally, these large plants require frequent pruning, which can be time-consuming and difficult in a greenhouse setting. The confined greenhouse space can also hinder their natural growth patterns, leading to an unmanageable garden.

For these reasons, giant trees and shrubs are not recommended for greenhouses and are better suited for outdoor planting, where they have ample room to thrive.

#2 Invasive Species

Invasive species can pose significant challenges in a greenhouse due to their aggressive growth patterns. These plants, such as mint, Japanese honeysuckle, and butterfly bushes, can spread rapidly, outcompeting other plants for nutrients, light, and space.

Their fast growth makes containment difficult, potentially harming the balance of a controlled environment and even impacting local ecosystems if they escape.

To prevent these issues, it's best to avoid invasive plants and instead choose non-invasive alternatives like basil, lavender, or native flowering shrubs that provide similar benefits without overtaking other plants in your greenhouse (1).

#3 Potatoes

Potatoes can be challenging to grow in a greenhouse because they are highly prone to pests like aphids and diseases such as blight. These issues can spread quickly in a confined space, potentially affecting other crops.

Additionally, potatoes have high nutrient needs, making them poor competitors when grown alongside other plants. This can result in stunted growth and reduced yields for both the potatoes and nearby crops.

Given these challenges, potatoes are best grown in open fields or garden beds, where there is better airflow and more space to prevent the spread of pests and diseases.

#4 Sunflowers

Sunflowers may seem like a beautiful addition to a greenhouse, but they can create significant issues for other plants. They release chemicals into the soil, a process known as allelopathy, which can inhibit the growth of nearby plants.

Additionally, sunflowers grow tall and can block essential sunlight from reaching smaller plants, impacting their ability to thrive. This combination of chemical interference and shade makes sunflowers a poor choice for greenhouse environments.

#5 Corn

Corn is not an ideal choice for a greenhouse due to its tall growth, which can cast shade over other plants and limit their access to essential sunlight. Additionally, corn shares common pests with tomatoes, such as corn earworms, which can quickly spread and damage multiple crops.

Corn also requires a lot of light and space to thrive, making it difficult to manage in the confined environment of a greenhouse.

#6 Water-Hungry Crops

Water-hungry crops can be problematic in a greenhouse due to the risk of excessive moisture, which can lead to root rot and fungal diseases. These plants require consistently moist conditions but can quickly become unhealthy without proper drainage and careful water management.

In a greenhouse, maintaining the right balance is crucial to prevent issues caused by too much water. Examples include celery and certain herbs like mint and cilantro, which thrive in moist conditions but need consistent monitoring to avoid waterlogging.

For instance, mint grows vigorously in damp soil, but without proper drainage, its roots can become waterlogged, leading to decay. Using raised beds or well-aerated soil can help mitigate these risks while keeping the plants healthy.

#7 Crops Requiring Pollinators

Crops that rely on pollinators, such as bees and butterflies, can struggle in a greenhouse due to the lack of natural pollination. Without these pollinators, fruit and vegetable yields may be significantly reduced, leading to poor harvests.

While it is possible to achieve pollination manually using a brush or by shaking the plants, this can be time-consuming and labour-intensive. Alternatively, you can introduce bumblebees or use self-pollinating plant varieties to help overcome this issue.

#8 Plants That Compete for Resources

Certain plants compete for resources like nutrients and space, leading to weaker growth and reduced yields. For example, radishes and broccoli compete heavily for nutrients, making it difficult for both to thrive in a confined greenhouse setting.

Similarly, marigolds can compete with legumes like beans for nutrients and space despite their reputation as natural pest deterrents. To make the most of these plants, it’s better to plant them separately or strategically place marigolds away from beans.

#9 Onions and Garlic

Onions and garlic may seem like good greenhouse crops, but their strong odours can inhibit the growth of nearby plants, especially in tight greenhouse conditions. The sulfur compounds they release can negatively affect the growth of more delicate crops like beans, peas, and certain herbs.

While it’s possible to grow onions and garlic in a greenhouse, they should be planted away from these sensitive plants to prevent any negative impact on their growth.

Instead, consider planting them near compatible crops like carrots or tomatoes, which can benefit from the pest-repelling properties of alliums.

Frequently Asked Questions

Can I Grow Tropical Plants in a Greenhouse?

Yes, you can grow tropical plants in a greenhouse in the UK, but it requires careful climate control. Ensure the greenhouse maintains high temperatures, humidity, and adequate lighting to mimic tropical conditions. Using heaters, humidity monitors, and supplemental lighting can help achieve this.

Proper ventilation is also essential to prevent mould or pests.

What Are Some Tips I Need to Know When Planting Tomatoes in a Greenhouse?

When planting tomatoes in a greenhouse, ensure they receive ample sunlight and are in nutrient-rich soil. Maintain a temperature between 18-24°C and provide good air circulation to prevent mould. Water at the base to keep the soil moist but not waterlogged.

Prune side shoots and support plants with stakes or cages.

Conclusion

In conclusion, choosing the right plants for your greenhouse is essential to creating an environment conducive to healthy growth.

By avoiding plants that outcompete others, attract pests, or require excessive maintenance, you can prevent turning your greenhouse into a breeding ground for issues like disease and poor yields.

Careful planning and informed choices will ensure a productive and thriving greenhouse that supports a wide variety of crops.

Sources -

1. https://www.houseoftents.co.uk/greenhouse/planting-guide/