Normally freshwater aquariums are imitations of nature and for this you must know that in nature the substrate is warmer than the water.
A very important thing you should know is that in nature the substrate is warmer than the water, this is because the heat from the sun is absorbed by the substrate which means that in time the substrate is getting warmer than the water.
In almost all cases the temperature difference is about 1 °C, at first view this value might seem very low but it is enough to generate the convection currents which move the water through the substrate, in this way roots have access to nutrients.
The cycle in nature is quite simple: Sun warms the substrate → Substrate warms the water → The warmed water is rising → Water with a lower temperature drops, bringing nutrients to roots.
To mimic nature many manufacturers have created a heating system with cables that warms the substrate, usually it is called Undergravel heating cable.
If you do not use a undergravel heating system the water from your aquarium will be warmer than the substrate, just opposed to nature. In this case plant roots will not have access to the nutrients.
On the market you will find many systems to heat the substrate with or without thermostat and my advice is using one with thermostat is more expensive but you have more control over the substrate temperature, a good example is Dennerle Thermo Control Bodenfluter with Duomat Evolution DeLuxe.
More details can be found on Aquarium substrate heating.
You should know that undergravel heating cables and fine sand are not compatible because the water is not circulating in the fine sand.