Growing tomatoes in containers is almost always an adventure. It can be incredibly rewarding or flat out disastrous. Sometimes epic failures can happen for reasons beyond your control like tomato blight or a ridiculously wet or cold summer. However, if you avoid some common mistakes, you 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 in the container, the more it 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 the key to tomato success. Too much water and the plants drown—too little could cause blossom end rot, when the tomatoes turn black on the bottoms. Inconsistent watering can also cause blossom end rot, split tomatoes, and stressed plants. A critical component 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 soil moisture first. To do this, push your finger into the soil about an inch or two—about down to your second knuckle. Add water if the soil feels dry to the touch at your fingertip. Another method to check moisture is to pick up the pot. If its weight feels unusually light (or top heavy) for its size, moisture content could be low.
Don't forget drainage—make sure the pot has drainage holes in the bottom to allow excess water to drain out. Pot feet are also a good idea if the pot is located on a patio or non-porous surface. Add water until it drains out of the bottom of the pot to ensure that water has reached roots growing near the bottom of the pot. Another great way to control water in your containers is to use a self-watering container, such as a grow box. You may want to try the Earthbox or the GrowBox brands.
Too Little Water
The amount of water your tomato plant requires depends on a few things, including the weather. Wind, heat, humidity, the size of the pot, and the kind of potting soil you use affects how often you need to water. By mid-season, a large tomato plant might need watering at least once a day and sometimes twice. Also, when you water, make sure to really soak your plants—if you give them just a sip, the water will only penetrate the top layer of soil. When you apply water, water the soil and avoid wetting the leaves because wet leaves can lead to fungal diseases. Don't bother with water crystals, they are expensive and tests have shown that they aren't particularly effective.
Planting several plants in one pot might seem like a good idea, but it usually is counterproductive. Unless the pot is tremendous in size (like the size of a raised bed) plant only one tomato plant per pot. To get an idea of minimum size, one tomato plant can be successfully grown in a large reusable grocery bag, which is the minimum size per plant.
Not Enough Sun
Tomatoes are sun-lovers and require 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 receives. Determine actual sun exposure, either with a watch or a sunlight meter, before you position your pots. Also, the amount of sunlight that reaches an area can change dramatically over the growing season, so re-check every week or so to make sure nothing is obstructing access to sunlight.
Along with lots of sunlight, tomatoes like warm temperatures. While it might feel like you're getting a jump on the season by planting tomatoes early, they will not thrive until temperatures are consistently warm. If you do want to get a jump on the season, you can either cover your tomatoes with 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.
Tomatoes are heavy feeders and need to be fertilized regularly if you aren't using a pre-fertilized potting soil. Most potting soils contain very few of the nutrients 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. You have 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 might consider adding a diluted fish emulsion/seaweed liquid once every week or two, or calcium, either in the form of lime or liquid calcium. Black areas at the bottom end of a tomato indicates the fruit could be suffering from blossom end rot, which can be caused by irregular watering and/or a lack of calcium in the soil.
Staking or Caging Too Late
Waiting too long to stake or cage a tomato plant is a chronic mistake. Tomatoes grow quickly, and it is best to stake or cage them at planting time before they grow large and unwieldy.
Container Grown Tomatoes. Penn State University Extension
Growing Tomatoes in Home Gardens. University of Minnesota Extension