When Can I Plant Tomatoes in an Unheated Greenhouse?

Greenhouses are valuable tools for extending the growing season of various plants, including tomatoes. Growing tomatoes in an unheated greenhouse can be a great way to get an early start on your garden, but how do you know when it's safe to plant them? The answer is not straightforward, as it depends on several factors, including your location, the variety of tomatoes you're growing, and the conditions within your greenhouse.

Determine Your Planting Zone

The first step in determining when to plant tomatoes in an unheated greenhouse is to identify your planting zone. Planting zones are determined by the US Department of Agriculture (USDA) and are based on the average annual minimum winter temperatures. Knowing your planting zone will help you figure out when the last frost date typically occurs in your area.

Last Frost Date

The last frost date is the date after which it's safe to plant cold-sensitive plants outside. It's essential to know your expected last frost date to avoid planting tomatoes too early, which can result in stunted growth or death due to cold damage. For most areas in the United States, the last frost date falls somewhere between late March and early June. However, it can vary widely, depending on your location, so check with your local cooperative extension service for an accurate prediction.

Choosing the Right Tomato Variety

Not all tomato varieties are created equal when it comes to temperature tolerance. Some varieties are better suited for cooler temperatures and can handle frost much better than others. If you plan to grow tomatoes in an unheated greenhouse, you should select a cold-tolerant variety that's well-suited for your climate. Some varieties that are known for their cold tolerance include 'Siberian,' 'Latah,' 'Green Zebra,' and 'Stupice.'

Conditions Within Your Greenhouse

Finally, you must consider the conditions within your greenhouse when deciding when to plant tomatoes. While an unheated greenhouse will protect your plants from frost, it won't necessarily provide enough heat to germinate seeds or encourage growth. If nighttime temperatures drop below 50°F, tomatoes won't grow or produce fruit. Therefore, you'll need to monitor the temperature inside your greenhouse carefully and take steps to increase heat, such as using a heating pad or heat lamp.

Conclusion

In conclusion, the best time to plant tomatoes in an unheated greenhouse depends on your location, the variety of tomatoes you're growing, and the conditions within your greenhouse. Your best bet is to consult with your local cooperative extension service for advice specific to your area. But if you select a cold-tolerant tomato variety, wait until after your expected last frost date, and take steps to increase heat inside your greenhouse when necessary, you'll likely enjoy a bountiful tomato harvest in no time!