I plan to cycle a new 20 gal tank and would like to put in some sword plants immediately. Will the ammonia and nitrite cause harm to the plants?

No I believe the plants will be ok, it will help keep your levels lower also so it is probably a good idea.

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Tank
125g african cichlids making it work
75g rimless Reef in progress

It's actually a good idea to put plants in during a cycle, go for it!

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"Being ignorant is not so much a Shame, as being unwilling to learn".-- Benjamin Franklin

Should be no problem. Plants consume Ammonia, Nirtite, and Nitrates as fertilizer. I have always thought than plants should be in a cycling tank as they help to temper fish toxic Ammonia and Nitrite llevels. Plants also provide food (Algae and Protozoa) for cyling fishes.

I would watch Swords in a 20g. At Best I would go with a Ozelot or Kleiner Bar Sword. An Amazon sword, which is the easiest to find out there, will get way too big for a 20g. Will take a little while to get there, but once it gets to growing well it is usually not all that far behind. Here is a pic of one in my 29g...it is touching front, back, and left side:

Hello dar...

Actually, just the opposite. Aquatic plants prefer soft, acidic water. They'll use the ammonia, nitrites and eventually nitrates for food. That's why planting the tank well during the cycling process is recommended. The more plants, the better.

I personally like Water wisteria, Water sprite and Pennywort. These are fast growing stem plants that really thrive in the tank during the cycling process because there's a lot of dissolved pollutants in the water.

Just a couple of thoughts, to consider or not.

B

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Low Tech Water Building Keeper

I'm thinking about amazon swords because they always were the hardiest plants for me, and grew well. I never had luck with other plants. I do worry about one growing too big. I like to pull them out and split them, but I don't think the fish would like the mess that would stir up.
I would like to try other plant species if I felt confident they would survive. Also I'm going to float a bunch of anacharis, that's always easy.
My water is hard alkaline in southern california.