House geraniums -- what's wrong here?

I have some geraniums on a wonderfully bright windowsill and they are blooming like crazy but the leaves of the plants are getting smaller and smaller. I have them planted in quart size glass canning jars to keep the overall size of the plant small. I do fertilize them with a 1/4 dose of violet fertilizer about every other time I water them. Is that the problem? Do I need to ease off any sort of fertilizer and give them a chance to rest and regroup for awhile?
I don't want to burn them out even though it's lovely to have such color on the windowsill. Will a geranium eventually die off like an anuual should or is it possible to keep them for years and years?
Giselle
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I'll take a far-out guess and say it might have something to do with heat. Glass jars will concentrate heat. And, of course, they have no drainage, right? I assume you haven't drilled holes in the bottoms. You didn't mention the chemical analysis of your plant food, but it these were my plants, I'd eliminate the obviously wacky mistakes first, and then move to the mundane. Get them into regular flower pots WITH HOLES IN THE BOTTOM, and trays that ARE NOT ATTACHED, so if you overwater, you can take them to the sink and let them drain.

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