Short of growing space? Consider this

I've not tried this idea myself, but in theory it sounds very practical. I personally would try to get hold of square containers rather than round to maximize soil mass as well as ensure they sit more stable on the gutter and can't be knocked off too easily (eg cat/dog help). The water source would also include occasional additions of liquid feeds like comfrey tea, manure tea, etc - but strained carefully so nothing down the line gets plugged.  

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It looks like a practical way to keep your plants watered. To make it look a little better, you could use fiberglass planters rather than buckets. What would be even better is if there was a way to fill up a large barrel or bucket of water to feed the gutter system so everything will get plenty of water if you go on vacation. I like how organized the gutter garden makes things.