Congratulations! It’s your first year growing tomatoes in a greenhouse. You’re in for a treat—extended harvests, faster growth, and plants protected from unpredictable weather. Hurrah!

But greenhouse gardening comes with its own set of challenges. You might be wondering:

• Wait, there’s no rain to water my plants.
• How do bees find my tomatoes if they’re hiding in a greenhouse?
• Is the air circulation good enough in there?

This article is an extension of another post where I focus on a common issue when growing in-ground in a greenhouse: crop rotation (which is extra important with nightshades like tomatoes!)

In this guide, we’ll look at 8 other common things gardeners miss when growing tomatoes in a greenhouse that can hinder their harvests.

Let’s get to it.

Quick Overview: 8 Common Greenhouse Tomato Mistakes

1. Poor Airflow: Leads to fungal issues.
2. Low Ceilings: Causes trellising problems.
3. No Rainwater: Lacks a natural water source.
4. Overhead Watering: Encourages soil-borne diseases.
5. Cramped Plants: Limits growth and health.
6. Pruning Challenges: Makes maintenance difficult.
7. No Cover Crop: Reduces soil health.
8. High Temperatures: Stresses the plants.

Keep scrolling for detailed solutions to each problem.

1. Poor Airflow: Leads to Fungal Issues

Poor airflow in your greenhouse can create a humid environment. Think about it - the covered sides protect the tomatoes from a lot of weather, but the typical wind that outside plants receive is helpful.

This lack of wind is the perfect breeding ground for fungal diseases like powdery mildew and blight. These diseases can be devastating to the health and yield of your tomato plants.

Here’s What to Look For:

• Yellowing leaves with white, powdery spots.
• Leaves curling or drooping for no apparent reason.
• A musty smell in your greenhouse.

Here’s How to Solve It:

• Install Ventilation Fans: Ensure there is consistent air movement. This will reduce the risk of fungal diseases.
• Open Windows and Doors: On warm days, increase natural airflow.
• Prune Lower Leaves: This helps with air circulation around the base of the plants, reducing humidity levels. It’s like giving your plants room to breathe.

2. Low Ceilings: Causes Trellising Problems

Tomato plants, especially indeterminate varieties, can grow quite tall. Low ceilings limits your trellising options. It can be tempting to just let them sprawl - but they’ll soon become tangled and difficult to manage.

Here’s What to Look For:

• Tomato plants bending or collapsing under their own weight.
• Trellising structures hitting the ceiling.
• Reduced plant vitality due to lack of vertical space.

Here’s How to Solve It:

• Use Shorter Varieties: Go for determinate or dwarf tomato varieties.
• Adjust Trellising Techniques: Use horizontal trellising to guide the plants. This can be akin to training a vine to grow along a fence.
• Prune Regularly: Keep the plants more compact - look for pruning opportunites a couple times a week

3. No Rainwater: Lacks a Natural Water Source

Tomatoes in a greenhouse miss out on the natural watering provided by rain, which means you'll need to be diligent about manually watering them.

Here’s What to Look For:

• Dry soil, especially during hot weather.
• Plants showing signs of water stress (wilting, curling leaves).
• Uneven soil moisture levels.

Here’s How to Solve It:

• Install Drip Irrigation: Gives your plants consistent moisture without overwatering.
• Use Mulch: Helps retain soil moisture and reduces evaporation. Imagine mulch as a protective blanket for your soil...cozy.
• Monitor Soil Moisture: Regularly check and adjust your watering schedule as needed.

4. Overhead Watering: Encourages Soil-Borne Diseases

One benefit of greenhouses is that no rainfall means no wet leaves. This reduces your chance of disease.

But, no rain means that unless you have drip irrigation, you’re likely watering with a hose and a spray nozzle. Watering from above can splash soil onto the leaves, spreading soil-borne diseases like fungal spores, and creating an environment conducive to disease development.

Here’s What to Look For:

• Leaf spots and blight symptoms.
• Soil splashed onto lower leaves.
• Uneven watering patterns.

Here’s How to Solve It:

• Switch to Drip Irrigation: Directly waters the roots and keeps foliage dry.
• Water at Ground Level: Use a watering can with a narrow spout.
• Water Early in the Day: Allows foliage to dry quickly and reduces humidity. Think of it as giving your plants a morning shower, setting them up for a fresh start to the day.

5. Cramped Plants: Limits Growth and Health

Most of us don’t have a huge array of greenhouses like how they do on market garden farms. Every inch of growing space is precious. This may tempt us to reduce the spacing on our tomato plants to get a few extra in.

However, overcrowded plants compete for light, water, and nutrients, which can stunt growth and increase susceptibility to diseases.

Here’s What to Look For:

• Plants shading each other out.
• Stunted growth and smaller fruits.
• Increased incidence of pest and disease problems.

Here’s How to Solve It:

• Space Plants Adequately: Follow recommended spacing guidelines for tomatoes - typically at least 18 inches.
• Prune Regularly: Remove excess foliage to improve light penetration.
• Use Grow Bags: Helps manage space more effectively. Think of containers as giving each plant its own personal space.

6. Pruning Challenges: Makes Maintenance Difficult

Pruning is essential for healthy tomato plants, but greenhouse conditions can make it more challenging due to confined spaces and dense growth.

Here’s What to Look For:

• Overgrown, tangled plants.
• Difficulty accessing the center of the plants.
• Increased humidity and poor air circulation.

Here’s How to Solve It:

• Regular Pruning Schedule: Prune suckers and lower leaves regularly.
• Use the Right Tools: Sharp, clean pruning shears can make the job easier.
• Train Plants Early: Guide them to grow in a manageable way from the start, much like training a puppy—early guidance makes a big difference.

7. No Cover Crop: Reduces Soil Health

Usually we’re trying to grow food year-round in a greenhouse. I totally get it - fresh salads, carrots, and kale throughout the winter is valuable for anyone who wants to be more self-sufficient.

But without a cover crop, soil in greenhouses can become depleted of nutrients and organic matter. This leads to poor plant health and reduced yields.

Here’s What to Look For:

• Soil compaction and poor structure.
• Decreased plant vigor and yellowing leaves.
• Increased pest and disease problems.

Here’s How to Solve It:

• Plant Cover Crops: Use hairy vetch to improve soil health during off-seasons. It can fix nitrogen in the soil, and has been shown to reduce diseases that commonly plague tomatoes.
• Rotate Crops: Avoid planting tomatoes in the same spot each year. Here’s my guide to crop rotation with tomatoes in a greenhouse to help.
• Add Organic Matter: Regularly incorporate compost or aged manure into the soil. Consider this as feeding the soil a tasty snack of microbes and organic matter. Yum.

8. High Temperatures: Stresses the Plants

Greenhouses can trap heat…

But this means that sometimes plants get higher temperatures than you anticipate on sunny spring days, or are extra impacted by heat waves. This heat stresses tomato plants, leading to poor fruit set, blossom drop, and lower yields.

Here’s What to Look For:

• Wilting leaves during the hottest parts of the day.
• Blossom drop and poor fruit set.
• Sunscald on fruits.

Here’s How to Solve It:

• Shade Cloth: Use to reduce the intensity of sunlight. Think of it as giving your tomatoes a beach umbrella.
• Ventilation: Open windows and use fans to cool the greenhouse.
• Water Wisely: Water plants in the early morning or late evening to help them cope with heat.

Conclusion

By paying attention to these common mistakes and implementing the suggested solutions, you can ensure a healthier, more productive greenhouse tomato garden. Remember, consistent care and attention to detail will reward you with bountiful harvests. Happy growing!

Still excited about how a greenhouse can benefit your tomato yield? You might be interested in my buyer's guide to greenhouses for your backyard garden.