My Pond's Gone Green! Four Things To Know About Algae

The first thing to know about algae is, it is completely normal for new ponds to go green. In fact, they are much more likely to go green than not. Every year we are contacted by people who have recently built a new pond and put in some plants, and are puzzled and worried because, about two weeks later, the water has gone very green, and/or slimy green strands have appeared everywhere. It's important to remember that the majority of ponds you see, either in the wild or in people's gardens, have been established for a while, and have gone through this stage. Green water - the 'pea soup' effect - is caused by single-celled or tiny individual algae suspended in it, while blanketweed, stringy algae, or slimy algae, are caused by filamentous algae. Filamentous algae is usually attached to surfaces, such as the pond sides or plant leaves, but it may be free-floating. 

The second thing to know about algae is, you cannot keep it out of your pond by anything that you do, and you do not 'catch' it from anywhere as such. Another common thing that people say to us is, that they bought plants from another nursery and 'caught' algae from them. This widespread belief is one reason that we carefully wash and clean all our plants before they go out so that they have no algae on them, but in truth it makes no practical difference. What has happened in this case it that the person has built a new pond, or cleaned out an old one, and then bought plants from a less careful nursery, which arrived with visible algae on them. When their pond goes green, or filamentous algae appears, soon afterwards, they naturally assume this has come in with the plants. In fact any body of water that is new, or disturbed in some way, will inevitably go green, and the timing is coincidence. The algae is triggered by nutrients being released into the water column - nutrients either from the tapwater used to fill the pond or from adding plants in pots of soil. More rarely, nutrients are released from disturbing sludge on the bottom when cleaning. Algal cells can indeed come in on plants, but this is true even if the plants appear completely clean, and they can also come in on the wind, on birds' feet, on aquatic insects, etc. Algae cannot be kept out, except by the constant use of strong disinfectants in the water, as is done with swimming pools. 

The third thing to know about algae is, it is mostly harmless. (If you are interested in wildlife it's also helpful to remember that it is the primary basis for the aquatic food chain). Gardeners strongly dislike the appearance of algae and green water, and often think they have to quickly remove it in some way. Very intense growths of algae can indeed cause some water quality problems (mostly low oxygen levels or pH swings) which can harm pet fish, but these are not common and can usually be avoided by circulating the water with a pump. If you don't have fish in your pond, then algae is more of a cosmetic issue. The most likely problem it can cause is that it may cut out light to your deep-water plants, and delay their establishing. It isn't likely to kill them, but if it is very extreme and prolonged and the plant is small and vulnerable, there is a small chance of this. If your plants seem to be really struggling for light, green water problems can be avoided by raising the plant higher in the water so that light can better penetrate to it. Stringy algae problems can be avoided by physically pulling it off the plants as much as you practically can. There are also numerous over-the-counter algae treatments you can buy from aquatic shops, from barley straw to phosphate blockers - these are generally harmless to plants and aquatic life and work well provided that they are regularly re-applied.

The fourth thing to know about algae is, it will usually go by itself. Or rather, it will settle down to a naturally low level that most people are happy with. Once a pond has been set up for a while, the nutrients tend to leave the water column and become incorporated into the substrate and the plant material, and the pond clears. Invertebrates that feed on algae start to move into the pond and it falls to the bottom in their droppings, or it simply exhausts the food supply and dies back. In a pond without any plants at all, it will persist but at a lower level. In a pond with a good amount of plants, once these are established they compete with the algae for light and nutrients, and keep it under control. Provided that there isn't a continual influx of new nutrients into the pond (from ground run-off, fish being fed, waterfowl droppings, or dredging of sludge) it should then stay clear. It's common for an algal bloom to appear in spring each year - algae's ecological niche is to get going in spring quicker than the plants can - but this clears as the year advances and the plants catch up. Algae may bloom again in an established pond after a disturbance, such as adding new plants or cleaning the pond, but this bloom is usually much shorter lived than the initial one. 

If you do have a continual flow of nutrients into the pond, from fish that you are feeding every day, from waterfowl, or from ground run-off, you can fight the algae by choosing faster-growing plants that will compete better with it. It will also help if you thin their excess growth regularly and remove it. When you remove plant foliage from the pond and compost it, you are also removing the nutrients locked up in its structure (rather than leaving it to die off and re-release these nutrients back into the water). If this is not enough and algae is still a problem, speak to your aquatic shop about ultra-violet units, phosphate blockers and biological treatments.