Growing Tomatoes in a Greenhouse
Written by Matt W (Greenhousestores) on 2nd Feb 2016.
Those who are new to greenhouse gardening often think that glasshouses are best suited to growing flowers, herbs, and perhaps cool season crops like leafy green vegetables. The common belief among novices is that “picky” summer vegetables like tomatoes and peppers can be started in a greenhouse in late winter or early spring, but must be moved outdoors once the weather warms up.
That couldn’t be further from the truth. While it may be easier for a newbie to tend a tomato crop planted outdoors, growing tomatoes in a greenhouse isn’t difficult once you get the hang of it – and nothing beats having a year-round supply of delicious, home-grown tomatoes. Growing Tomatoes outside can be quite challenging given the UK's unreliable climate and weather conditions. Growing tomatoes under glass will always yield far more fruitful crops, year in, year out.
Before starting, it’s important to ensure that you have the proper environment for growing tomatoes in your greenhouse. Unless it’s already summer and you’re planting for a fall crop, you will probably need to supplement the natural light inside your glasshouse with grow lights timed to operate 12-16 hours per day (high-pressure sodium lights are the best choice because they facilitate the growth of tomato flowers and fruit), and you may need to add heaters and timers to maintain proper indoor temperature. If possible, night-time temperatures between 15 and 18 degrees and daytime temps between 22 and 28 degrees are optimal; heat mats placed under the plants can help as well. Finally, good air circulation is important to maintain constant humidity and prevent the spread of airborne plant disease.
Ready to grow? Here’s your guide to growing tomatoes in a greenhouse.
When to Plant Tomatoes in Your Greenhouse
When to plant? Hey, you’ve got a greenhouse – you can plant anytime you want! Seriously, though, most gardeners want to have an outdoor tomato crop during the summer, so we’ll start there.
The first step is finding the “last frost date” for your immediate area, which will tell you approximately when the danger of winter frost has passed. With that information, you can count backward to the proper date to start your tomato seeds. There are a number of online resources which allow you to enter your region or town and find out the proper last frost date to use. You should start your seeds six weeks before that date, and plant the seedlings outdoors between a week and ten days after the last frost date; that’s to avoid the danger of a very late surprise frost, and also because tender plants appreciate it when you give the ground an extra week or so to warm up. In the UK, the normal period for sowing seeds is between the start of March and the end of April. You can check out our greenhouse growing guide for advice on the timings for planting and harvesting of all manner of plants and vegetables.
Of course, the real advantage of having a properly-heated glasshouse with adequate lighting is that you don’t have to be a slave to the seasons and can sow your seeds at any time. That doesn’t mean you can just throw some seeds into some soil and be feasting on juicy red tomatoes on New Year’s Day, however. All tomato plants require plenty of attention, but those grown during the cold seasons need extra care – and that begins with choosing the right plants. "Determinate" varieties, often called bush tomatoes, are hardier and better suited to late summer, fall and winter planting because their shapes provide better protection for the fruit and take up less space, making them the best choice for most home greenhouses where space can be at a premium and temperatures can vary. Determinate tomatoes produce their crops all at once rather than fruiting throughout their growing season (like the indeterminate varieties which prosper outdoors during the summer). Staggered planting dates throughout the cold weather months can ensure a continuous supply of greenhouse tomatoes. Indeterminate varieties can be grown in a glasshouse as well, but will grow much higher and require strong support for the vines; many feel cherry or plum tomatoes are the best indeterminate for indoor growing. In general, indeterminate tomato plants will produce a bigger crop, but determinate varieties will give you more fruit per square metre. We'll get into the specifics of planting seeds and tending plants shortly, but be aware that growing “off-season” tomatoes in a greenhouse during winter’s shorter days (with less natural sunlight) and in colder weather demands greater attention to maintaining proper temperatures and humidity, and positioning plants so they receive as much sunlight as possible (supplemented by grow lights as needed).
Growing Tomatoes from Seeds
There’s certainly less effort needed to start your indoor tomato garden with plants from the garden centre, but it’s more rewarding to grow tomatoes from seeds and watch the first seedlings emerge. If you’ve never done it, it’s worth the extra time just to experience the thrill of seeing the small plants grow and thrive. You’ll also have a much greater choice of varieties when growing from seeds.
One of the most important decisions you’ll have to make comes well before you see fruit, shoots or even the first sprouts – it’s choosing the tomatoes you plan to grow. We’ve already mentioned the difference between determinate and indeterminate varieties; the amount of room you have in your greenhouse and the type of fruit you prefer should both be considered when making this decision. Most seed suppliers and online sites clearly label the best choices for greenhouse growing; a few commonly suggested varieties are Roma VF, Tumbling Tom and Red Alert for bush plants, and Alicante, Gardener’s Delight and Shirley for indeterminate plants which will be cordoned (more about that later in this article).
Now that you’ve selected seeds, it’s time to sow them. The process is the same whether you plan to keep them indoors in pots, move them to grow bags, or transplant them into the ground inside your greenhouse or outdoors. You’ll initially sow the seeds in pots, so let’s start there.
Growing Tomatoes in Pots
Some gardeners start their seeds the way that commercial operations do, in the small cell packs that are sold to consumers. There’s nothing wrong with that, but you’re better off using small 7.5-10 cm tall pots, wide enough to let the seedlings spread their roots. Fill the pots almost completely (leave about 1 cm of space at the top) with soilless potting mix, seeding or multipurpose compost, and place a few seeds on top near the middle of the pot. Most of the seeds will germinate, so don’t put too many in each pot – three to five seeds should do it. Cover them with a thin layer of soil mix or compost and press down gently so the seeds are completely in contact with the soil. Don’t forget to label the pots with their variety and starting date.
Now place the pots in an area of your greenhouse which receives at least four hours of direct sunlight per day. The more sunlight and the warmer the environment, the faster the seeds will germinate; the grow mats and grow lights we’ve mentioned will be a big help. You can even cover the pots with plastic to keep the heat in. If you take the plastic shortcut, though, be sure to remove the plastic as soon as you see the first sprouts. Otherwise, the chances are good that your plants will suffer from a lack of air circulation, contract the fungal infection known as “damping off” disease, and die.
Don’t overwater your seeds because they will rot in soggy soil. When the topsoil becomes dry, just sprinkle enough water into the pots to moisten it. You should see seedlings within about two weeks, and in six to eight weeks they should be large enough to be transplanted into their own, larger pots. During that growing period, be sure to provide good air circulation for the seedlings with an open window or a small fan, and if the greenhouse is cold at night, consider using horticultural fleece to warm the small plants.
It’s time to transplant the seedlings when they’re 2-3 cm tall and have developed a few leaves. Place each into a separate pot, handling them by their leaves and making sure the roots go as deeply into the compost or potting mix as possible. Once a plant has grown to about 20 cm it is ready to be transplanted again, into its “final” pot, a grow bag, or the ground. If the final destination is outdoors, don’t move plants them until it’s warm enough – usually mid-May or later in the UK – and be sure to harden them off first.
For tomato plants which will remain in pots for the entire season, don’t skimp on the size. If a plant outgrows its “final” pot, it’s a major project to transplant it again. Smaller varieties like bush plants or cherry tomato vines should go into a six litre (or bigger) pot, while indeterminate varieties will need one that’s at least ten litres. There should be many small holes in those pots (if you don’t use specially-designed containers like Air pots, you can drill your own) so the roots can breathe. Some pots are more like decorative garden ornaments, shaped like pagodas or drinking troughs. All are ideal providing the size is such that the plants can root down and spread adequately.
You will of course position your tomatoes where they have room to grow and receive as much sunlight as possible, but also be certain that you place the pots where you can provide support for the plants. Even bush tomatoes can benefit from supports like cages, but vining varieties will require cages, trellises or stakes.
Growing Tomatoes in Grow Bags
Grow bags became quite popular in the 1970s as a way to cultivate tomatoes along the inside borders of a greenhouse without “using up” the soil in the ground or exposing plants to diseases or pests which might be present in the soil. They fell out of favour for a while, but are once again popular – and a terrific way to grow tomatoes in a glasshouse (or outdoors, to make the most of a short growing season).
Ready-made grow bags the easiest to use, and also contain compost designed to work without the drainage normally provided by the holes at the bottom and sides of pots. If you’re planting directly into bags of compost or making your own bags, though, you’re best off puncturing the bottom of the bag in a few spots because your plants will most likely need the drainage help. There are also grow bags made of porous material, which provide the best drainage and aeration possible for your tomato plants. To get the bag ready, use your hands to break up all clumps of compost which may have formed inside, then open the pre-cut slots at the top (or cut your own, if necessary). Soak the pot containing your small tomato plant in water for an hour to prevent root damage and then use a trowel to create space for your plant and its root ball. The hole should be deep enough so the top of the root ball sits completely inside the bag and can be covered with a thin layer of compost. Gently firm the plant and the topsoil, and water well. Remember, the plants will get much larger and their roots will spread inside the bag. A 60-litre grow bag shouldn’t be home to more than two plants, and a 75-litre bag should hold three at most. A grow bag support frame can be a worthwhile investment, as it slips underneath the bag and holds canes or poles used to support the plants as they grow.
Compost for Growing Tomatoes
There are a number of different compost products on the market. Seeding compost is light on nutrients while potting compost has the nutrients your plants will need as they grow, and multipurpose compost can be used for both seeds and plants. The ideal approach is using the “proper” compost for each stage of your plants’ development, but that can get expensive. A cheaper approach is to purchase grow bags – even if you don’t plan to use the bags – and use the compost that’s inside the bags. It’s the same quality of compost that most commercial growers use, and if they do fine with it, you can as well. If you have your own compost pile, don’t hesitate to use it; many experts swear by homemade compost for their tomato crops.
The term “cordon” is often used interchangeably with the term “indeterminate,” but in reality, cordon refers to the stem of an indeterminate tomato plant when it grows without extra branches. How does a plant grow without branches, you ask? That requires some actual gardening. First, though, a very short biology lesson.
The important structural parts of a cordon tomato plant are the roots, the main stem (which grows from the roots), and the leaf stems (also called “trusses”) which grow out from the main stem. The leaf stems are where the flowers grow and the fruit develops. As indeterminate varieties grow, they send out many small side shoots (sometimes called “suckers”) above or below the leaf stems. Those shoots, if left alone, will grow into new “main” stems or create even more side shoots, using up much of the nutrients the plant needs to create fruit.The best crop of tomatoes, therefore, is produced when the plants are pruned (or cordoned) to ensure that they only have one main stem. That is done by pinching off those small side shoots regularly; checking each plant once a week will let you catch the side shoots early enough that they’ll snap off easily in your thumb and forefinger when you bend them. As long as you don’t accidentally remove or damage the leaf stems above the shoots, you’ll end up having a single tall vine with productive leaf stems growing lots of tomatoes.
A cordon tomato plant probably will never grow tall enough to let Jack climb up to find the Giant’s gold, but it will keep on growing and growing – and diverting nutrients to the new growth – unless you stop it. That’s easy to do by a process known as stopping tomatoes (sometimes called topping tomatoes).
Once a plant has grown four to six leaf stems (the decision depends on how tall your greenhouse is and how well the plant is flourishing) it’s time to stop its upward growth by cutting the main stem at a point two leaves above the top leaf stem. From that point on, all of the plant’s energy will be used to grow those beautiful red tomatoes on your existing trusses.
Determinate varieties do not need to have side shoots removed or to be stopped.
Feeding Your Tomatoes
There’s nothing complicated about feeding your greenhouse tomato plants. Simply use a nitrogen-rich liquid fertiliser every one or two weeks (read the label on the container to determine the optimal feeding schedule) while the plants are first growing, and then switch to a high-potash, high-potassium “tomato plant” fertiliser after the first tomatoes have started to set.
Two other quick feeding facts: first, if you have a sick plant it should not be fed; starve the plant until it starts to recover. Second, liquid fertiliser leads to an accumulation of salts in the compost. Skip a feeding twice during the plant’s life cycle and give lots of extra water instead, to wash out some of the salts.
Watering Your Tomatoes
A general benchmark is that a greenhouse tomato plant needs a little over one litre of water per day, more in hot and sunny conditions, less in cool and cloudy conditions. Plants appreciate daily, light watering much more than being drenched every once in a while. The latter will lead to cracking or splitting in the tomatoes’ skins.
The best way to know if your tomato plants need water is to examine the soil and the plants. The soil from the top to a depth of 5 cm should be moist but not soggy and the leaves should not be wilting. Dry soil and wilted or dark green leaves are a clear indication that the plants aren’t getting enough water. On the other hand, soggy soil and light (almost yellow) leaves are signs that you need to cut back on your watering.
Tomato Diseases and Pests
When you grow tomatoes in a greenhouse, they’re less susceptible to blight than ones growing outdoors. However, there are two pests which are quite common to glasshouses and can do great damage to your tomatoes.
- Red Spider Mite: You can’t see these mites (which love the protected environment of a greenhouse) with the naked eye, but you’ll know there’s a problem if you see mottling, bronzing or speckling on the top of your plants’ leaves. Immediately turn down the heat and continually mist the underside of the leaves (where the mites nest) with water; then get a predatory mite called phytoselius persimilis, which will eat the red spider mites, at your local garden centre. Avoid pesticides, because they won’t kill the red spider mites but will kill the “good guys” instead.
- Whitefly: Whiteflies show up in the spring and start as tiny (1.5 mm), scaly crawlers before becoming small white moths as adults. It’s best to be proactive because whiteflies are quite common in glasshouses. There’s a two-step process: in early April introduce the parasitic wasp known as Encarsia Formosa, which will eat the nymphs, into the greenhouse. Then later in the month, hang fly-catching sheets near your plants to catch the adults. Most whiteflies are resistant to pesticides; the sprays that do still work can also be absorbed by your tomatoes. Other issues which affect outdoor tomatoes can be a problem in greenhouses as well. Growing marigolds near your tomatoes will help attract hoverflies if you have an issue with aphids, but many chemical sprays will also do the job. Mosaic virus is also a major issue in the UK, and is distinguished by leaves which become misshapen and a mottled yellow in colour. Remove the leaves from plants and from the greenhouse immediately, be sure not to touch any other plants until you’ve washed thoroughly, and give the affected plants plenty of food and water – they should recover much of their strength.