Tomatoes grown in containers need the same amount of sunshine as those cultivated in the ground-at least 6-8 hours per day. They also need even amounts of water, so make sure to check the container's moisture levels daily, especially in hot weather. As for your plant getting spindly, all tomato plants benefit from regular pruning. Snap off the suckers (shoots that appear between the main stem of the plant and the petioles (stems of the leaves). Proper pruning will help channel the plants energy into producing fruit rather than leaves and improve the air circulation around the fruits. Fertilizing with too much nitrogen can also result in spindly plants.
Containers dry out quickly. Keep well watered and much top of container. Also fertilize often because if you are watering correctly (by watering till water is coming out of the bottom or side drain holes ). The fertilizer gets washed out. Also if you are growing a determinate variety (growth is pre determined ) you do not need to prune suckers or any other pruning other than removing dead leaves.
Sounds like you need more nitrogen in the soil. Tomatoes are heavy feeders and expect supplementary fertilizer during the growing season. Be sure to provide lots of water in hot weather. Tomatoes use a lot of moisture.
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