A greenhouse provides perfect conditions for growing tomatoes throughout much of the year. Unlike planting outside, when you grow tomatoes inside of a greenhouse, you’re using fresh soil each year which is very beneficial for the long-term health and growth of the tomatoes.
Using soil in a greenhouse provides the plants more nutrients while also avoiding much of the pests and diseases outside, ensuring your plants make it through the year and provide a great harvest. Once the plants die off, you can recycle the compost as a soil improver, meaning little goes to waste!
There are two main options when growing tomatoes in a greenhouse – grow bags or pots. Both have their pros and cons, so it’s often down to your individual preferences, so let’s take a closer look at each method to see what might work best.
Grow Bag vs Pots
Many argue that pots are always a better option for growing tomatoes in a greenhouse because you control the quality of soil.
Grow bags offer obvious convince, with all the soil you need in one bag. Simply cut holes at the top, plant the tomatoes, and leave them to grow. They are also good at avoiding water loss from evaporation too.
However, the popularity of grow bags has resulted in a drastic reduction in quality. Grow bags are now smaller than ever and are often filled with less nutritionally dense compost than previously. Of course, good quality grow bags are available, just make sure to check their nutritional content to ensure they are suitable.
Also, if the grow bags do not provide enough soil it means there is less water retention. When soil cannot hold enough water, they’re more susceptible to drying out in hours, which could result in problems with the tomatoes like split skins and blossom end rot.
The only solution for this is do constantly water the plants throughout the day. Using larger grow bags is a good option for avoiding these types of issues, although you will need to water them regularly regardless – tomato plants love their water!
Water retention isn’t the only problem when there isn’t enough soil, as the tomato plants may also run out of nutrients before the end of the season. This means you’d need to supplement with liquid feeds
Pots don’t have many setbacks besides requiring a little more effort to plant. Providing the pots are the right size, most types of tomato plants thrive when potted inside a greenhouse. Also, make sure pots a washed before reusing them!
What size pots for tomatoes in a greenhouse?
As previously mentioned, choosing the right size of pot is important when growing tomatoes, as you need to ensure there is enough space for soil so that healthy roots can develop. In most cases, the bigger the pot the better, although the size of your greenhouse and the number of plants you want to grow with impact this. Read our guide on how to ripen tomatoes here.
Anything above 10” is suitable for planting tomatoes in pots, which holds around 10L of soil, with 12” being a great size capable of holding around 25L of soil. The more space the pot has the more water is retained, reducing the risk of soil draining too quickly and becoming dried out.
However, if you’re using pots to grow tomato seedlings then it’s a good idea to start much smaller, usually with 4” pots. This gives the roots lots of space to spread, so you can transfer the seedling to larger pots after 2-3 weeks of growing. If they outgrow their next pot, don’t hesitate in transferring to an even larger pot!
What size of grow bag for tomatoes in a greenhouse?
While their soil quality is often lacking, it’s easy enough to find good quality growing bags suitable for tomatoes. Standard growing bags have a 30-40L capacity, which is only a minimum size recommendation, as you want to go much bigger to ensure large tomato plants.
A good grow bag size is 60-75L of soil, which should give enough space for healthy roots and to hold enough water. There is enough space in these bags for 3 or more plants, but it’s a better idea to restrict them to just two if possible, as this gives even more soil for the tomatoes to grow in.
Another popular option with grow bags is to use a grow bag pots or old pot to increase the amount of soil available. Grow bag pots are basically bottomless plant pots that sit on the surface of the grow bag, allowing you to add even more compost on top.
You can make these yourself using old plant pots. Just cut the bottom of an old 8/9” plant pot and place it a few inches into the grow bag – just cut more of the bag away t make space. This lets you increase the soil volume without needing to buy purpose made bottomless plant pots.
Should you want even more soil for your tomato plants – which means bigger harvests and less watering – then you can stack two grow bags on top of each other.
Simply prepare the base growing bag as normal, then add a second bag that most of the base cut away, planting the tomatoes into the top bag. This basically doubles the grow bag size for even larger tomato plants!
This method is a good option if you can only source smaller grow bags or there is less space in your greenhouse.
No matter the size of growbag, make sure you properly prepare it before planting the tomatoes in. This includes cutting holes at the base for better drainage and breaking up clumps of soil. Always soak the tomato plant before transferring into a grow bag.
You may also want to invest in a frame that helps support the plants as they grow. If you need to heat your greenhouse during the winter month please check out our reviews of the best greenhouse heaters.