Depending on what types of plants and plant pots you are using, it’s generally a good idea. Why? Often to prevent fungus gnats and other pests from laying eggs to later infest your plants.
Top dressing, is usually a light coating of sand, small pebbles, and certain types of tree or orchid bark, on the surface of the medium where plant is growing up and outward. Where the stems meet your medium, where you cannot see the roots, that’s where you add your top dressing.
Just as I stated previously, with preventing fungus gnats from laying eggs, top dressing can also be used for decorative purposes, to give the appearance of your plant something extra.
It’s a good idea to top dress your plant, specifically if you have the option to bottom water your plant. For some that use sand, you can occasionally water the top, where the sand is, to make it a little heavier and this will make it even more difficult for bugs to lay eggs.
Generally, when bugs like fungus gnats encounter this, they’ll often go to another plant, without this issue. This is where you’ll need mosquito bits to counter this problem also, by making the “tea” or to have your plant medium mixed with mosquito bits.
Can this be a problem? It can, but mostly with planters that have no openings at the bottom. You may have to watch for how much water absorbs into the plants, because without the hole(s) in the bottom, you won’t know if you over did it, until you see that water surface, which will lead to having to dump that excess water. This will ruin your top dressing and it can lead to that mixing into the soil; which can be a good thing for extra drainage later on, but I digress. In this case, you’ll need to keep an eye on the plant, but to also check with a chop stick, or bamboo skewer, to check the soil for dryness, just to make certain it isn’t too dry for too long.
Sphagnum moss is also good for top dressing plants, and it’s good for absorbing water also. Just as the rest of the medium uses water, so does the sphagnum moss, but the moss can eventually liven up, and even turn green. When this happens, the dried moss can become live moss.
All of this is optional, but many people swear by it, so before you go about this, do a little research and practice, practice, practice, doing it right. What do I mean? Well, if the plant is very close to the soil/medium, then you may have to lift the leaves up, and put the top dressing of choice under those leaves. Also, when repotting, you can also add the top dressing, prior to adding the plant, by putting a pen or something slightly wider, like a marker, to make that hole, but only remove it, just before you will place the roots of your plant in that hole. This way, you won’t have to need to brush off the top dressing from the leaves of the plant very often.