Answer: It could be lots of things. My first guess would be soil. How is your soil? Is it very sandy, or heavy clay? Have you ever had the pH (relative acidity) tested? Do you garden in the same spot, year after year, without replacing nutrients and/or adding compost? Your plants may grow well in the spring, when temperatures are cooler and moisture is plentiful, but when summer hits, and the plants are stressed by heat, or too little (or too much) water.
Disease and/or insects can also play a role in stunting plants, so you'll need to inspect your plants carefully, looking for insects or insect damage, or signs of disease like leaf spots.
If you can find any particular symptoms that might help us identify the problem, feel free to submit another question to the web site and we'll try to be more specific.
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