Growing tomatoes in containers is almost always an adventure. It can be incredibly rewarding or flat out disastrous. Sometimes epic failure can happen for reasons beyond your control like tomato blight or a ridiculously wet or cold summer. However, there are some common mistakes that, if they can be avoided, will vastly increase your chances of successfully growing tomatoes in containers.
When it comes to tomato containers, bigger is better. The bigger your container, the more soil it will hold. The more soil you have, the better the soil holds water. Also, the more soil, the more available nutrients for your plants. Consistent water and food are two of the most critical elements for happy, healthy tomato plants and large harvests.
Too Much Water
Watering your tomato plants properly is probably the main key to tomato success. Too much water and the plants drown—too little and you get blossom end rot. Inconsistent watering can also get you blossom end rot, split tomatoes, and stressed plants. A critical thing you must do for tomato success (and the most difficult if you are using conventional pots instead of self-watering) is to keep the soil in your pots consistently moist—not wet, but damp.
Before you water, check if your soil is already moist. To do this put your finger into the soil about an inch or two—a good way to do this is going to your second knuckle. Add water if the soil feels dry to the touch at your fingertip.
Don't forget drainage—make sure your pot has large holes in the bottom so excess water can drain out. Pot feet are also a good idea if you have your pot on a patio or non-porous surface. Add water until it drains out the bottom of your pot. That way you will know that all of the roots, even those at the bottom of the pot, have gotten watered. Another great way to control water to your containers is to use a self-watering container, such as a grow box. You may want to try the Earthbox or The Grow Box brands.
Too Little Water
The amount of water your tomato plant needs will depend on a few things, including the weather. Wind, heat, humidity, the size of your pot, and the kind of potting soil you use will affect how often you need to water. By mid-season, a large tomato plant may need watering at least once a day and sometimes twice. Also, when you water, make sure to really soak your plants—if you just give them a sip, the water will only wet the top layer of soil. When you water, try to water the soil directly instead of the leaves because wet leaves can lead to fungus. Don't bother with water crystals, they are expensive and tests have shown that they aren't particularly effective.
Putting lots of plants in one pot may seem like a good idea, but it usually is counterproductive. Unless my pot is tremendous (more like a raised bed) I only put one tomato plant per pot. To get an idea of minimum size, I have successfully grown one huge tomato plant in a large reusable grocery bag and that's about as small as I'd go per plant.
Not Enough Sun
Tomatoes are sun-lovers and need full sun, which means that they need unobstructed, direct sunlight for 6-8 hours a day, no cheating or skipping. Many people chronically overestimate how much sun an area gets. Figure sun exposure out, either with a watch or a sunlight meter, before you position your pots. Also, the amount of sun that hits a spot can change dramatically over the growing season, so re-check every week or so to make sure there is nothing obstructing the plants' access to sunlight.
Along with the sun, tomatoes like warm temperatures. While it might feel like you're getting a jump on the season by putting your tomatoes out early, they will not really do anything until it is consistently warm. If you do want to get a jump on the season, you can either cover your tomatoes with clothes or plastic when it's cold or put them on carts and wagons and haul them in and out of an enclosed area (like a garage) until temperatures warm up. If going this route, don't forget to harden off your seedlings.
Starting Your Plants
Tomatoes are heavy feeders and need to be fertilized if you aren't using a pre-fertilized potting soil. Most potting mixes have very few of the nutrients that your plants require to grow and be healthy, so you will need to add those nutrients to the soil or supplement the nutrients already present if your mix is heavy on compost. There are many fertilizers to choose from, but some good options are an all-purpose, organic slow-release fertilizer or one designed especially for growing tomatoes or vegetables, which you can mix into potting soil. In addition, you may consider adding a diluted fish emulsion/seaweed liquid once every week or two, or calcium, either in the form of lime or liquid calcium. If you start seeing black at the ends of your tomatoes, you probably have blossom end rot, which can be from watering issues and/or a lack of calcium in your soil.
Staking or Caging Too Late
This is a chronic mistake for gardeners. It is easy to forget how fast tomatoes grow and, consequently, fail to stake or cage them until they are huge and unwieldy. It is much better to set up your cages or stakes before your tomatoes get too big.