Caring For Your Pond Plants Through The Seasons
Late autumn or as needed - Removing excess growth
Check your pond regularly during the growing season to monitor if any plants are spreading more than you would like. How fast pond plants grow depends on many things; the geographical location of your pond, the amount of sun it receives, the depth they are growing at, the size of pot, the type of soil, the amount of fertiliser, and the presence of competitor plants or herbivores. Because of this it is hard to predict exactly how a plant will grow. If plant material needs removing because it's spreading too far or because it's smothering something else, put on gloves and simply pull it out physically until you are satisfied with what is left. Garden scissors for snipping stems are very useful when doing this, but be careful of damaging pond liners. Less commonly, duckweed, blanketweed, or algae can cause problems for pond plants by stifling their growth and require removing. Duckweed is not a type of algae but a true tiny plant, and can be skimmed off the surface with a fish net. See our page here for dealing with blanketweed or algae.
If you are able to leave this job until late autumn, it will be more convenient, because it can be combined with other jobs and because it will cause less disruption to wildlife (see below). However, if autumn is a long way away and the unwanted plants are growing fast and causing problems, do the job then.
Late autumn - Protecting against frost and removing detritus
Some pond plants, such as Umbrella Grass or Arum Lilies, are slightly tender and cannot take their crowns being frozen hard. The crowns of these plants are generally kept just below the water in winter, so that the pond water itself protects them from frost. Check them in late autumn, before the first frosts in your area, and ensure the pot is deep enough in the pond that the ice will not reach the crown of the plant.
Removing detritus such as dead leaves from the bottom of the pond is not essential. However, if so much has built up that it is reducing the depth of the pond, or if you keep suffering from algal problems, it is probably wise to remove some of it. The ideal time to remove detritus from the water is mid to late autumn (around October) if you want to minimise disruption to wildlife. At this time most of the flying insects and other creatures will have finished their life cycles for the year, and any amphibians that might decide to hibernate at the bottom of the pond will not yet have moved in. Detritus is best scooped out with a fish net; nets are available from aquatic shops in a range of sizes. You can also use something like a plastic trowel or dustpan if it doesn't have sharp edges that might damage your liner. If possible, leave the water in place - draining and re-filling a pond will disrupt the balance and result in more algae when the weather warms up. The detritus will usually still have many small pond creatures in it - if you want to save as many as possible, placing it on a tray or similar that slopes downwards will cause the creatures to wriggle downwards into the water as it drains out of the detritus.
Late autumn to early spring - Removing dead vegetation
Most pond plants will die back completely for the winter, losing their leaves and stems. The only exceptions to this are one or two evergreen marginals such as Corkscrew Rush, and some oxygenators such as Spiked Water Milfoil. Some die back earlier in the year immediately after flowering, but most keep their leaves until the first hard frosts. Dead stems and leaves don't absolutely have to be removed, but removing them will make your plants look much fresher and tidier. It will also prevent them from falling in and decaying in the water. As mentioned above, leaves decaying in the water actually isn't a huge problem and does provide natural food for small water creatures, but it will make algae more likely, and can also look unpleasant.
This dead vegetation can be removed at any time between autumn and spring. If done in autumn there is the advantage that it will make the pond look neat and tidy over winter. However, there is the disadvantage that the fallen leaves and hollow dead stems of plants, if left in place, provide important habitat and shelter for invertebrates over the winter. You will have to make the decision as to which of these is the most important to you, and what is the most convenient time for you. It's worth noting though that dead vegetation will also shelter and protect pest invertebrates - if you've suffered from an unwanted infestation of beetles, aphids, caterpillars etc on any of your plants, removing dead vegetation as soon as possible will reduce their numbers the following year.
Don't remove dead vegetation later than early spring, as it can become too difficult to trim off the old material without inadvertently damaging the new shoots.
Early spring - Re-potting, dividing and fertilising plants
Pond plants should be re-potted or divided at least every two to three years, or sooner if the plant appears hungry or cramped in its pot. This may simply mean the plant being removed from its old pot and re-potted into a larger pot. Alternatively, if for reasons of space you need to keep it in the same size pot, the plant can be divided with a sharp knife into an appropriate size, the roots trimmed, and then re-planted in the same pot but with fresh soil.
The best time to re-pot plants is early spring, just as they start to grow. This ensures that any nutrients and fresh soil will begin to be used straightaway, for the greatest efficiency and the least leaching into the water. In most cases, this is also the best time to cut and divide plants, since they will cope more easily with this if they can begin to grow again immediately.
If you are not re-potting your plant but are simply adding some fertiliser to the soil in order to give it a boost for that year, this is best done in early spring as well. See our page here for advice on choosing pots, soil and fertilisers.
Late summer - Dealing with drought
Water shortages, drought, and hosepipe bans are becoming a greater concern. Contrary to what one might think, pond plants require less care during droughts than most garden plants do. If the pond's water level drops significantly, aquatic plants will survive, provided that the soil they are in never completely dries out. This is even true of things like waterlilies, which can survive being totally exposed to the air if their actual soil is still moist. The water level in most ponds can fall a long way before the soil of the plants dries out, meaning that the pond will not usually require much or any additional watering. (Although this will depend on its size, since very small ponds and tubs will naturally dry out more quickly). The plants will not like being out of the water if that is not what they normally do, and their foliage will look awful and grow poorly, but this will not kill them. You can therefore top up the pond only as much as you need or want to.