The first thing you need to know about Koi pond plants is whether or not you really need them.
They do involve a bit of work but they provide many benefits.
First of all they help keep your pond clean, balanced and algae free.
The plants in your pond feed on unwanted nutrients like fish waste and nitrogen.
These are the same materials that feed algae and allow them to flourish.
Since algae and Koi pond plants compete for the same food and pond plants are higher on the food chain, they starve off new algae growth.
The best combination for your Koi pond is: lots of live plants & good filtration & a minimal amount of fish = a healthy ecosystem...
AKA a clean pond! Some other reasons that you need Koi pond plants include:

1. They provide cover for your Koi, thus preventing things like sunburn (yes, Koi can sunburn!) and predation from birds.
   Birds, with their keen eyesight have no problem locating your colorful Koi and swooping in to scoop them out of your pond.
   
2. As already stated they help reduce algae problems.
   
3. Pond plants help lower the water temperature of the pond thereby also lowering the day-night temperature differential of your pond.
   This helps reduce environmental stress on your Koi.
   
4. Another benefit is that they help lower evaporation rates...
   
5. And they provide nice, shady areas for your fish to relax in.
   
6. The plants in your pond help also help keep your fish well fed.
   They do this in two ways.
   First, they trap some of the floating fish food, giving your Koi another chance at it before the pond skimmer gets it.
   Secondly, as Koi are voracious eaters, the plants themselves provide a "fresh food" source for them.
   No matter what you do you are going to have to plan on periodically replacing some of your plants.
   Your Koi won't be able to resist them!
7. Pond plants also provide excellent cover for Koi fry (Koi babies).
   While you may not plan on raising any babies they will still be born and having pond plants ensures that at least a few will survive.
   
8. Some of the plants in your pond will (and should be) oxygenator plants.
   The best plant for fast growth and best water filtration is the water hyacinth.
   The oxygenator plants are the backbone of what aquaculture is intended for.
   They filter contaminants out of the water and produce new oxygen.
   
9. Last but not least, the plant life in your pond creates a beautiful backdrop for your colorful and elegant fish! Don't underestimate the aesthetic beauty they will add to your Koi experience.
   

As you can see, there are many excellent reasons that you should invest your time and money in Koi pond plants!