Dividing Water lilies & Pond Cleaning

Ponds need occasional cleaning to keep them at their best, and water plants, just like garden perennials, need dividing occasionally to keep them healthy and prevent them becoming overgrown. I was forced to take this advice seriously this year, when the entire root mass of my water lilies detached from the bottom of the pond and floated en-mass to the surface.

My lily (Nymphaea ‘Gonnere’) is classed as a medium/small variety, listed as spreading 3-4ft across and needing at least 18 inches of water above the crown. My pond (which is actually two rigid liners, one each side of the false ‘bridge’) should be perfect. Each is 6ft x 4ft, and two feet deep in the middle, so should be able to accommodate one ‘Gonnere’ on each side… That is: as long as I divide them regularly.

My pond is ten years old, but has never been properly cleaned out. For many years our neighbours had a huge eucalyptus tree which dropped indestructible leaves all over our garden. Thankfully it has now been felled, but in the intervening years, a good percentage of the canopy ended up in our pond helping create a deep layer of sludge. This fertile compost encouraged the water lilies to romp away, their roots spreading well beyond their original baskets, until they colonised the entire base of the pond. I knew they were getting out of hand a couple of years ago when their leaves started growing above the surface of the water, but I kept putting off the inevitable.

This year I was forced to take action as the entire mat of roots (strangely, on both sides) floated to the surface, creating a horrible mess and causing the lilies to begin to yellow and die. The buoyancy was incredible, lifting several house bricks that had been used many years ago to keep the young plants submerged.

During a warm spell in September I bit the bullet, rolled up my sleeves and got stuck in…

Step 1: Lifting the lilies

The root mass was huge: nearly four foot square and a foot deep. And way too heavy to lift in one go. So, using a garden spade, I chopped each root ball into three or four pieces — taking care not to damage the liner in the process. Then, with a garden fork, I lifted each chunk out and placed it on a builder’s mixing-tray. Later, once drained, they went in the compost bin.

I kept the best chunks of water lily for replanting: a bucket-sized portion for each side with healthy looking roots and a nice fresh set of buds. I placed the good bits in a dumpy bucket with enough water to keep them from drying out.

While I was at it, I removed some of the other floating plants (Water soldiers) along with some of the marginal plants, old aquatic baskets, bricks and other junk. Again, I placed anything I wanted to keep in a bucket of water. Any plants that is. The bricks were perfectly happy on dry land.

Step 2: Cleaning out the ponds

Once the lilies were out, I partly drained the ponds, reducing the water level by a foot so that the marginal plant shelves just remained moist. Being a two-day job, I didn’t want the remaining marginal plants to dry out overnight.

I then used a pond net to drag the base of the pond, removing as much of the leaves and mud as possible. This was very messy and took a longer than expected. Finally, I skimmed the surface of both ponds repeatedly to remove all the floating detritus.

I did not fully empty and thoroughly clean the ponds. Partly, this was to preserve the marginal plants that have colonised the shelves so nicely (although I did remove some that had never performed well(, but also to keep some of the native microbes to help repopulate the pond hopefully helping it quickly restore its natural balance.

Step 3: Replanting the water lilies

I purchased two new aquatic baskets, larger than the originals (13in / 33cm across) and planted the healthy lily divisions that I had salvaged. I packed aquatic compost around their roots and topped off the baskets with gravel to prevent the soil washing away. The new plants looked really healthy and were much larger than newly-purchased plants (above, left). They should establish quickly.

I placed the baskets in the pond on up-turned plastic crates (above, right) to ensure the water lily pads ended up at the right height. I will lower the baskets gradually each year as the plants grow.

Step 4: Refilling the ponds

I topped up the pond with tap water. Rain water would have been better, but I didn’t have any at hand. Hopefully, because I didn’t empty and/or sterilise the pond, it will cope with the tap water and re-establish a healthy microbial community rapidly.

Once full, I skimmed the surface to remove any remaining muck. I actually found a sieve quicker for this job than the pond net as it had larger holes that did not become blocked so easily.

The lilies will take a while to settle in, but they look much happier. I only returned a couple of water soldiers to each pond.

The final result

As you can see, the pond is looking much more orderly than it did at the beginning. I guess it will be good for another decade!

2 thoughts on “Dividing Water lilies & Pond Cleaning”

  1. I have read that water lillies are toxic-did you have any problems using the compost from them? I ask because I am cleaning out my pond and wonder if I can use them as mulch in my orchard. Great photos in your article!

    • Hi Tom, No, I didn’t use the compost elsewhere. But they can’t be that toxic as fish and pond life thrive in the lily pond. I think you can put the compost on your orchard without any worries.


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