Answer: As a rough guide, healthy torenias are usually in the 6 to 12 inch range depending on variety, but the "Clown" strain, one of the most popular, is only 6 to 8 inches tall, so I don't know how large yours should be. However, your torenias should bloom steadily through fall, so apparently there is some sort of a problem.
Ideally, the soil should be rich, evenly moist (not soggy), and well supplied with organic matter such as compost for optimum growth. Well prepared, loosened soil also encourages quick rooting of new plants. As with all annuals, torenias appreciate additional nutrients such as those found in a balanced fertilizer. The rule of thumb is to work in 2 to 3 cups of a balanced fertilizer such as 10-10-10 per 100 square feet before planting.
I don't know how you prepared the bed, but one common reason for plants to yellow and stop blooming is lack of nutrients. You might try a balanced foliar feed such as Miracle-Gro or try scratching in some balanced fertilizer around the plants as a "side dressing" especially if you did not add any at planting time.
Another common reason is over-watering. The amount of water the plants need will depend on the make-up of the soil they are growing in, the amount of rainfall and the ambient temperatures. The rule of thumb for watering is an inch a week from either the sky or the gardener. A weekly deep watering is more effective than daily sprinklings. The best way to tell if they need watering is to stick your finger in the soil and see if it is damp or dry under the surface.
One more reason plants can look and poorly is insect infestation. You might want to take a very close look at the plants, especially the undersides of the leaves, just to be sure that is not the cause.
An organic mulch is useful for nearly all types of plants for a number of reasons including that it cuts back on the need for watering, suppresses weeds and it feeds the soil as it breaks down. The mulch however can rob your soil of nitrogen, so if you mulch be sure to supplement with some form of nitrogen.
Good luck with your torenias!
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