I'm at 6500 ft in ponderosa pines and struggling to produce tomatoes large enough to eat. They are currently about the size of a small ball and ripening. They are in my hot house getting as much care as possible. All tomatoes are growing well, flowering, and fruiting but staying small. I keep the door on the hot house open to keep it from overheating the plants. In past years I've kept them potted and on a southern exposure that gets sun most of the day but I never had tomatoes then either. They get watered routinely, fertilized with water soluble solutions (miracle gro) weekly and look great -- but I want tomatoes for eating and they just don't make the grade.
El Paso County Colorado
Thank you for your question.
You are over-fertilizing.
Tomatoes have a low nitrogen requirement. Under high nitrogen conditions, vines grow excessively large at the expense of fruit production. More correctly stated, tomatoes are a fussy nitrogen feeder. On soils low in organic matter, tomatoes typically run out of nitrogen in mid-summer, reducing yields and predisposing the plants to Early Blight. At transplanting, apply one to three applications (depending on soil organic content) of a water-soluble, “plant starter” fertilizer. This includes any of the water-soluble products like MiracleGro.
Temperatures in your greenhouse: Don't fertilize if temps are lower that 55 ºF. A week with daytime temperatures below 55 ºF stunts tomato growth, reducing yields.
Mid-summer – On low organic matter soils, tomatoes typically run out of nitrogen in mid to late summer. Yellowing of the foliage, starting with lower leaves, is the typical symptom of nitrogen stress. Low nitrogen in the plant allows Early Blight disease to spread like wildfire. Keeping nitrogen levels up in mid to late summer is a primary means of Early Blight control and significantly improves yields. Fertilize tomatoes lightly as the first fruits reach two-inches in diameter. Watersoluble fertilizers (such as MiracleGro) used according to label directions make a good summer fertilizer supplement. Make applications every two to four weeks, depending on soil organic content. 717-6 If using a dry granular fertilizer, apply 21-0-0 (ammonium sulfate) at the rate of one level tablespoon per plant. Sprinkle the granular fertilizer in a wide circle 12 to 20 inches out from the plant, and water in. Dry granular fertilizers can easily kill tomatoes if over-applied.