Plant baskets for pond plants are designed to allow good water movement and nutrient delivery to the roots without allowing the compost to escape and cloud the pond water. If you're careful when you lower your plants into the water, you shouldn't need to line the baskets with hessian, but the option is there if you're a 'belt-and-braces' water gardener! Some pond plants are supplied in solid-sided pots like normal garden plants - if possible you should re-pot them in a proper plant basket to reduce the risk of to roots compacting or rotting in anaerobic conditions (where you get that rotten-egg swampy smell from stagnant water!).
What size basket?
Bigger is better! The bigger the basket, the more compost, nutrients and growing space you're providing, so the better your plant will look up top! Make sure your pot is at least twice the size of the current root system or rhizome. Contact us if you need help working out how much you need.