For a low gravity planet I'm working on I thought of airborne rootless plants, but there's a problem, how would they get their nutrients? Plants on earth use their leaves for photosynthesis and their roots to get nutrients and water, so is there a way for these rootless plants to get them without roots? I imagine these plants evolved pretty early during the colonization of land, and that their ancestors either had no roots or small ones, if that helps.
Different types of plant can adapt to lack of soil better than others. There will be a variety of methods they adopt to cope.
One, as mentioned in other answers, is making do with only those nutrients that can be pulled from air. As pointed out, there are already plants that do this. My favorite example is the type of plant that lives in the canopy of some rain forests. All it ever gets is rain, air, and the occasional leaf from its host tree.
Not all plant life cycles can function on this limit. Some will need other nutrients, particularly at the time of flowering and seeding.
Another aspect is that at least part of the eco system will float or fly around with the plants. Insects, birds, and probably other animals will find it advantageous to fly. Or at least be able to move somehow along with the plants. There will probably be some analog of birds, but also of many other animals that adapt in different ways. Perhaps by having a large hydrogen gas filled buoyancy bag.
If you have not, you might want to read John Varley's Gaea series. In it there is a species of living dirigible. The largest and oldest is called Whistle Stop, and he is something like 2 km long. Since he is full of hydrogren he has a certain attitude about flame and other sources of ignition such as lighting.
Some plants will have structures that catch dead leaves and other detritus from other plants. These will provide a minimal amount of nutrient. There may be relationships where a particular type of plant requires something from another plant, and in exchange drops its dead leaves in a way useful for the other plant. Maybe the very-spiky plant grows around the roots of the very-leafy plant, and they both are better off.
Some plants will behave like the Venus fly trap and catch smaller animals for their nutrients. This could either be something they do regularly or only when they flower and produce seeds. In a floating situation, there might be sticky nets to catch birds, or long sticky strings extruded to catch insects. They might adapt methods to predate on the animals that have adapted to fly. If the hydrogen floating critter has a skin that can be punctured, it might be susceptible to thorns. Just as an example.
Some plants will form a symbiosis with animals. Perhaps the animal's droppings are enough. Perhaps the plant requires other waste such as cast off feathers and such. Or perhaps when the animals die their bodies wind up used by the plants. In exchange the plants might provide shelter, hiding places, and possibly some food for the animals.
Birds might fly to the land and bring back mud to make nests. After they are through with the nest it might be fertilizer for the plant they built in. If there were several thousand birds in a floating tree they might provide a substantial amount of soil. The bird and the tree might well adapt to each other so that the bird nests become more useful to the plant. And the tree adjusts to accomodate the birds.
Perhaps there is a species of bee that happily brings nutrients that can only be found in soil. So the plant they fertilize will be much larger and healthier, so the bees get fed by the plant. The bees might be able to bring things like phosphorous or potassium that are difficult to get except from soil. Perhaps the plants then produce lots of nectar for the bees. The bees might also aggressively protect their home plant, preventing flying cows from eating it.
Bromeliads aka "air plants" use roots only to hang on. They absorb water and nutrients from the air. That is what these plants are doing on the wire - there is no water or nutrients there. That is how your flying plants do it too.
For a flying "plant" I might imagine a lichen would be best suited for that lifestyle. Lichens grow on bare rocks and like bromeliads obtain nutrients and water from the air.
Or you could riff on the mangrove, and have your floating plants evolve from seeds / propagules.
Mangrove seeds are technically called “propagules” because unlike most other plants’ seeds, mangrove propagules germinate while still on the tree! This is an adaptation that helps then to grow rapidly upon falling to the soil below once they are ripe.
Your plants had ancestors with airborne floating seeds that then evolved into airborne propagules. Then some of these propagules complete the life cycle without ever coming down to land. They are airborne plants!
There are plenty of scheme on how floating plants could float here on WB stack, Good luck with your world.
If your low-gravity planet will allow objects as massive as plants to float around, there will also be floating water, floating dirt, and floating nutrients
Nutrients are, from one point of view, a recycling of life. A plant draws from the soil, lives, and then dies and decays, returning those nutrients to the soil for the next round of plants.
Your world will be no different, you've simply shifted where it all takes place. And it's important to realize that you can't have plants floating around the sky and not have water and soil floating around in the sky. If an object of mass X can float in the sky, then all (and I mean it, ALL) objects of mass <=X will also float in the sky.
This means that water and soil is floating around in the sky. Your atmosphere is rich in nutrients! Plants draw from the nutrients found in the floating water and soil, live their lives, and then die and decay, returning those nutrients to the sky.
As for roots...
From this perspective, I might think that your plants won't be rootless. On the contrary! Roots would be longer and might contain lots of macro-cilia-like tendrils designed to capture nutrients as "stuff" floats by the plant. Roots become even more important in areas where nutrients must be drawn from a larger area. So I vote that your plants will definitely have roots. Really long roots that look like someone's hair after rubbing it with a balloon.
Your real problem is sunlight
Floating plants means floating water and floating soil. This means that below a certain altitude, competition for sunlight increases. The floating water and soil will cause problems. The soil will obfuscate the light. To a lesser degree, so will the water. But curiously, the water droplets (or globs, remember, mass <=X!) will act as lenses focusing light and causing the potential for things to burn. Focal lengths are important, but this increases the wondrous complexity of your ecosystem!
But that all-important sunlight must somehow be provided. This means either your plants have evolved to thrive in lower sunlight, or the sun is brighter (world is closer to the star, star is bigger, etc.) so the comparative sunlight at your ideal altitude is Earth-similar. This also could be fun because it means the soil at higher altitudes is warmer and the water vaporizes to create fog and clouds. An entirely different biome.
But let's assume that it's your world and you declare that water and soil won't be floating around regardless the issue of mass
In that case I propose that you need plants that operate a bit like whales, but in reverse. They need a reason to drop down to where the water and soil is to capture nutrients.
Once again, roots are important. Something needs to drag on the ground/water-surface to capture nutrients.
But why would a plant do this? Let's assume your plants capture more than just nutrients. Let's assume (somehow) that as a by-product they also capture helium (or some other lighter-than-air compound). The helium pushes them back into the air because even on a low-gravity world, mass will pull to the ground. Something must keep the plants afloat. But you're dealing with biology, not synthetics, and so there's no way to perfectly contain the helium. It leaks out (through respiration or chemical diffusion or it's used to move the plants around... you know... herbal flatulence...). This cycle returns the plants over and over to the ground where dragging roots pick up nutrients and water.
I like soil/water in the air better. It feels more believable to me. But an idea like this could also be believable. After all, whales must surface for air periodically.
Again, it's a problem already solved by algae thriving in the open waters of the oceans and lakes, for which nutrients are captured while they float in the current.
You won't get something the size of a sequoia in this way, but nevertheless it will always be plants.
Just provide your organisms with ciliae which can increase the capturing area (and also better catch the wind) and the problem is solved.
If you want airborne plants I think a HIGH gravity planet would be much more likely to have such a flora. The reason being is that on a planet with high gravity the air particles would be more closely packed together making the air behave much like liquid. In this conditions the plant would float in large groups (vegetal clouds) steering the thick air with brush like structures that would gather water and nutrients- those nutrients might come from volcanic eruptions that would spread minerals in the liquid-like air. Also when that plants decay and die, they would also spread their minerals throughout the air.
Such plants would be ironically less likely to evolve on low gravity planets because you get the opposite effect- the air particles are more spread and few and far between therefor much less likely to be able to take nutrients from the air.
If you still want some sort of flying plant on a low gravity planet you cold make plants that develop saplings high in altitude. Those sapling than develop a method of hovering through the air in order to spread large distances to a new location where they would go to maturity. You could make so that all this saplings are usually released at once and hover very slowly creating this vegetal "clouds".