This post isn't so much DIY advice as it is preventative maintenance.
Never ever never put REAL potted plants on a hardwood floor. So far as I'm concerned, the ideal thing to brighten up any room with a hardwood floor is an artificial tree that you never need to water.
The problem with putting a REAL potted plant on a hardwood floor is that if you ever accidentally over water it, then you stand a good chance of staining the hardwood floor very badly.
...read on for the details...
The reason why is that plants have "tannins" in them. (see note below on what tannins are) It's tannins that give many woods their distinctive colour, such as in the case of red oak, redwood, cedar, southern yellow pine, mahogany, etc. It's also tannins that give plant extracts their colour and in some cases taste. The reddish colour of tea and the astringent taste you get when you bite into a grape seed are due to tannins in the tea leaves and grape seed.
Tannins react with iron ions in water to form something called "iron gall ink" which is a black material that was used in Europe for centuries as a writing ink.
So, if you ever see a round black stain on a hardwood floor, (which are about as common as a barking dog), what you're seeing is the remnants of an accidental over watering of a potted plant. What's happened is that someone overwatered a plant so that the excess water leaked out onto the floor. In passing through the soil, that ink picked up iron from the soil which then reacted with the tannins in the wood to form a black primitive ink, and that black ink is what's causing the stain.
To prove this info is correct, just go to the Canadian Wood Council's web page at:
Wait for the page to load completely as it may not respond properly before it's fully loaded.
Position your mouse cursor over the "Publications" link on the right top corner of the page, and you should see a drop down list. Click on the "PDF Publications" link in that drop down list.
On the Free Publications page, scroll down to the section entitled "Durability" and download the 8 page brochure entitled: "Discolourations on Wood Products: Causes and Implications". You will need Adobe Reader installed on your computer to open that .pdf file. (Note that the Canadian Wood Council represents the interests of the forest products industry in Canada. The "Forintek" group is a university of British Columbia based research group that provides technical advice to the Canadian Wood Council and forest products industry in Canada. The above brochure was prepared because customers are often reluctant to accept wood that has discolourations on it because they are concerned that those marks are due to mold or are caused by wood eating insects. They don't want to use that wood in their homes or to build homes because they're concerned the mold or insects could spread throughout the house.
On Page 5 of that brochure it talks about "iron stains" in wood as follows:
Lumber is sometimes discolored with ironstain – this happens when iron particles react with phenolic chemicals in the wood, leaving behind black iron tannates (a common ink pigment). Iron can come from steel wool, filings, lubricants containing metal fines, from invisible iron particles where the wood has rubbed on steel rollers or chains, or even from airborne particles, for example from the brakes or from the wheel-on-rail friction of railway cars (called travel stain). Diagnosing iron stain can usually be done by spotting a dilute (~3%) phosphoric or oxalic acid on the stained part; the acid breaks down the iron tannate into colorless iron salts, and the ironstain is decolorised.
While this brochure is referring to the black stains that might be seen on construction grade lumber, it applies equally well to any wood that has tannins in it being exposed to iron (typically iron ions in solution in water.) Exactly the same chemical reaction that causes "ironstain" in spruce studs will cause iron gall ink staining in hardwoods.
And, even though that brochure suggests that you can remove these stains with a mild solution of phosphoric or oxalic acid, an ounce of prevention is worth a pound of cure. Better to not have the potted plants on the hardwood floor in the first place. (Ungelled phosphoric acid is typically found in bathroom cleaners because phosphoric acid will cut through soap scum like a hot knife through butter, but won't attach chrome. Gelled phosphoric acid is typically found in toilet bowl cleaners, but it wouldn't work as well in it's gelled form because it wouldn't be asborbed into the wood as well as ungelled phosphoric acid. Oxalic acid is sold as "Wood Bleach" in hardware stores. It's typically used to remove the yellow discolouration that occurs on wood in the first few weeks of being stored outdoors and exposed to the elements.
So, if you want to keep a hardwood floor looking good, don't over water any plants so that the excess water runs out onto the hardwood floor. Better, in my view, is don't have plants on that floor in the first place so that accidental overwatering will result in a stain.
You don't need to read the rest.
This is the "Note below" on what tannins are:
A "phenol" is nothing more than a benzene ring with at least one hydroxyl group (-OH) bonded to it, like this:
Even if there's more than one (-OH) group on the same benzene ring, it's still just a "phenol" as in the case of gallic acid:
But, once you have more than one benzene ring with at least one (-OH) group bonded to it, then you have a "polyphenol", as in the case of tannic acid:
(Tannic acid is really just a central glucose (sugar) molecule with four gallic acid molecules hanging around.)
Obviously, there are very many different kinds of "polyphenols". It's estimated that over half of the mass of a tree's leaf consists of polyphenols. All of the various kinds of polyphenols in plants are collectively called "tannins". And it is these tannins that react with iron to make a primitive black ink called "iron gall ink".
And for sure you don't need to know this: The origin of the word "tannin" is that it is these kinds of chemicals (polyphenols) that were used in the leather TANNING industry to convert animal hides into leather.