Oxygenating plants or oxygenators are plants that, well, provide oxygen to the water.
Water without oxygen is a pretty smelly mess or stagnant.
While much of the oxygen is absorbed from the water surface and the interaction between the air and water (which is why moving water is a good idea
in a pond as it increases the water available for oxygen absorption) plants play a huge role in creating healthy water and preventing algae.
The difficulty of course is that there are always plants that overlap a bit depending on where or how they are grown. Ill describe these crossover plants in the text.
Rule of Thumb for Adding Floating Oxygenators
The rule of thumb for adding floating oxygenating plants to your pond if you do not have a pump or water fall working, to keep your pond algae-free.
1) calculate the surface area of your pond
2) calculate how much of that surface area is taken up by floating leaves such as water lilies
3) subtract #2 from #1. So if your pond surface area is 200 square feet and you have three lilies taking up 10 square feet each (3X10) then you have 30 square feet of floating leaves. 200 minus 30 equals 170 square feet of uncovered surface.
4) Divide the resulting number (170 square feet) by 3 to give you the number of floating oxygenators you require in the pond (56 in our example). Note that this does not apply to water hyacinths.
Note that the problem is mostly a spring problem that occurs before the leaves of lilies have expanded fully, the floating hyacinths have not yet exploded into growth, and water nutrients are readily available to algae.
Ponds without Pumps
Ponds without pumps really want to cover at least two thirds of their surface with leaves to reduce the algae buildup. See the article on algae control for other suggestions.