A Garden Expert Warns to Never Grow These 5 Plants in a Greenhouse – It'll Waste Your Time, Space, and Seeds

Adding a greenhouse to my garden is the best thing I've done. It instantly gave me more growing room, especially when it comes to starting flowers early and growing veggies. But, I've quickly learned you can't grow absolutely everything in a greenhouse – in fact, there are some plants that should be avoided at all costs.

It can be easy to get lost in the hundreds of beautiful greenhouse ideas out there, but it's important to remember the aim is to make it a productive space, as well as an aesthetically pleasing one. From fast-growing annuals to cool-season crops that just don't get along with the warmer temps and limited room of a greenhouse, it's best not to waste your time (and seeds) trying to grow them here – you'll just end up with dying seedlings and struggling plants.

To help you avoid this costly and frustrating mistake, I've asked a garden expert for her top five plants to never grow in a greenhouse. Here's what she said.

5 Plants You Should Never Grow in a Greenhouse

While there are several vegetables to grow in a greenhouse and many beautiful blooms to start early in a greenhouse, there are also multiple plants that will actually be at a disadvantage if you try to grow them in this controlled environment.

Instead, their size, growing habit, and needs demand outdoor planting.

1. Sunflowers

Fast-growing flowers are usually ones you should avoid growing in a greenhouse because of the rate at which they grow. It often means they don't need to be started indoors, and can be sown directly outdoors when the warmer temperatures arrive.

For this reason, Siobhan Shaw, sustainable gardening expert and founder of Growing to Give, says to avoid sunflowers.

'They need wide-open space, strong airflow, and full sun. Inside the structure of a greenhouse, they can quickly outgrow the space and compete for light,' she explains.

With many popular sunflower varieties reaching more than 10ft, a greenhouse is simply too restrictive.

Not only this, but sunflowers also develop a deep taproot and don't transplant well, so they're best sown in their final position in a garden border or pot.

'I prefer planting sunflowers around greenhouses instead. They make beautiful seasonal windbreaks, support pollinators, provide bird feed, and help soften the appearance of a functional building,' Siobhan notes.

Explore sunflower variety seeds for your yard at Burpee.

Siobhan Shaw

Siobhan is the co-founder of Growing to Give, a dynamic global nonprofit dedicated to transforming community-focused food security and agriculture through sustainable farming and gardening practices. Siobhan is a passionate advocate for women's empowerment in agriculture, mentoring women who aspire to careers in the field. Siobhan also continues her decades of podcasting as the host and co-producer of the health and well-being podcast titled Cancer Bites.

2. Radishes

Radishes are also on the list of plants to never grow in a greenhouse because they aren't equipped for the warmer temperatures this controlled environment offers.

'Radishes are fast-growing, cool-season crops that thrive in outdoor beds,' Siobhan notes.

'Because they mature in just 15-25 days, they rarely require the season extension a greenhouse provides,' she adds.

Not only is it unnecessary to grow radishes in a greenhouse, Siobhan notes it can also result in failure.

'Greenhouse heat can cause bolting (premature flowering) or reduce the quality of a radish.

'In a typical growing season, multiple outdoor plantings can be seeded in succession for continuous harvest,' she advises.

Find radish seeds to plant in your veg patch on Burpee.

3. Potatoes

While growing potatoes in containers is super easy and very common for space-saving gardening, Siobhan says using greenhouse for these crops can be a waste of valuable space.

'They're inexpensive staple crops that store well and perform efficiently outdoors,' Siobhan says. 'These versatile cool-season crops do not require protected space to produce reliably,' she adds.

There are so many ways to grow potatoes, including growing potatoes from a potato and even growing potatoes indoors if you don't have a garden.

But, if you are fortunate enough to have a yard with a greenhouse, Siobhan says to maximize its use with the plants that need it most.

'It is often better reserved for high-value crops that benefit more directly from controlled conditions,' Siobhan says. 'I suggest keeping potatoes in outdoor beds and pots instead,' she adds.

You can use these seed potatoes from Amazon to grow your own.

4. Cacti

This one is more ornamental rather than a plant you're growing from seed, but no matter how beautiful they look on your greenhouse shelf, avoid growing cactus plants here.

It can be easy to assume cacti will perform well in a greenhouse because its a suntrap, with plenty of light and warmth – just what these desert plants love. But, Siobhan warns to also keep in mind the humidity of a greenhouse.

'Many greenhouses retain too much humidity for desert plants. Cacti require dry air, excellent drainage, and strong ventilation,' she explains.

Greenhouses can become very humid because they're designed to trap heat, which is why it's important to ventilate your greenhouse and provide good airflow to the plants inside.

Yet, even with good ventilation, a greenhouse is made to be a warm, humid environment for optimal seed growing. It's far too much moisture for cacti, which thrive in arid conditions.

'Excess moisture can quickly lead to base rot and cause cactus plants to fall over.

'Unless a greenhouse is specifically managed for low humidity, cacti are usually better suited to outdoor desert climates or very dry indoor settings,' Siobhan says.

Alternatively, there are several tropical indoor plants you can use to decorate your greenhouse, including the moisture-loving Boston fern (available at Walmart).

5. Melons, Squash, and Pumpkins

Melons, squashes, and pumpkins are all large vining plants that demand a lot of room, not ideal for a greenhouse.

'A single plant may produce only a few fruits, yet the vines can extend 20 to 30 feet or more under ideal conditions,' Siobhan says.

'I have seen pumpkins grown in extremely limited soil space on our community projects, where vines traveled extensively and still produced large fruits.

'The growth was impressive, but it also demonstrated how much room these plants require,' she adds.

But, you should be aware that these are still plants to protect from frost, just not with a greenhouse.

Instead, use methods to protect vegetables from frost, such as using a horticultural fleece (like this from Amazon) and mulch.

FAQs

Can You Grow Tomatoes in a Greenhouse?

Yes, it's often ideal to grow tomatoes in a greenhouse, especially if you want an earlier crop. Greenhouses provide the warmth, light, and humidity to encourage tomato plant growth. A greenhouse also offers protection to tomato vines from harsh wind and blight, where they may otherwise experience damage in an exposed position outdoors.

As well as knowing which plants to never grow in a greenhouse, there are several greenhouse mistakes to have on your radar. This includes placing it in the wrong spot in your garden, hindering your plants' ability to grow.

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