Is Pond Water Good for Watering Plants? (Pros & Cons Explained)
Routine pond maintenance will typically involve a water changes, even for ponds with efficient filtration systems. This can amount to as much as 10 – 50% of the pond’s total volume, especially if you keep larger fish species, such as koi. At some point, a pond’s ecosystem will benefit from clean freshwater as nutrient loads accumulate. Even the most advanced recirculating systems in aquaculture make use of tools to strip water of nutrients, only to reuse the treated water once more. This highlights the need to conserve freshwater, which is a limited resource after all!
After a water change, you may find yourself wondering what to do with the gallons of water you’ve just removed from your pond. If you have a garden, you’ll instinctively look around and see a world of potential. Why not use pond water for your pond ornamentals, flowering plants, your whole lawn, and even your vegetables and herbs instead of letting it go to waste? Understandably, you may find yourself curious about the nutrient composition of pond water, how it may benefit your plants, and the possibility of any negative effects.
Each pond is different in the sense that it produces a water profile unique to its very own ecosystem. The types of nutrients and minerals, along with their compositions, are a result of your pond’s diversity, biomass, choice of fish feed, wild visitors, pond plants, climate and location, and more. The factors are endless! The good news is that, more often than not, pond water is safe for your prized flora. To fully gauge its potential for use, it’s important to understand what it contains, as opposed to freshwater from other sources.
Nutrient Profile of Pond Water
Unlike many large bodies of water in the wild, ornamental ponds are closed ecosystems. There’s no element of connectivity that ensures their water supply becomes mixed or naturally ends up elsewhere. Apart from rainfall, a pond can only rely on its filtration system for a semblance of access to refreshed water. This causes nutrients – sourced from fish waste, aging plants, naturally present microbes, pond additives, and surrounding run-off – to accumulate in the water.
Naturally, these nutrients contain both inorganic and organic elements, such as nitrogen, oxygen, phosphorus, and carbon (the building blocks of life). In ponds, the elements we are most concerned with are called macro-nutrients because they are present in large quantities. Typically, your pond should have balanced concentrations of these, as natural inhabitants will regulate their availability. Aside from the aforementioned nutrients, it will also have minute quantities of calcium, potassium, sulfur, and important micronutrients (e.g., iron, cobalt, magnesium, chlorine, vitamins, etc.). These quantities tend to vary depending on aeration, sediment types, fertilizer, and food.
Ponds that contain lots of fish will generally have higher nutrient concentrations, especially nitrogenous compounds. Fish waste or scum is perhaps one of the most concerning causes of nutrient build-up, as it contains ammonia and nitrites. Even in very low concentrations, these compounds can be toxic to fish. With beneficial microbes and ample pond aeration, however, these can be assimilated or reduced (via the nitrogen cycle) to non-toxic compounds.
Fertilizer Potential of Pond Water
Due to the rich nutrient profile of pond water, it can serve as organic fertilizer for plants. Plant fertilizer will typically contain nitrogen and phosphorus, which are vital for proper plant growth. As pond water has relatively high concentrations of these elements, it is often called “fertile water”. It provides the double benefit of providing both the necessary moisture and vital nourishment for plant growth. Pond water can significantly enrich soil, not only in individual pots and ornamental gardens, but even in farms that generate a large crop yield.
Historically, both natural and manmade ponds were fundamentally important sources of water for both domestic and agricultural use. Today, pond water continues to be treasured and has even been shown to be effective as irrigation water in paddies. Pond water, when “clean”, can increase plant productivity and is considered a friendly and economical alternative to fertilizer. In some remarkably exceptional cases, however, it can be detrimental to plants by introducing toxic conditions or pathogens.
When Might Pond Water Be Harmful to Plants?
Although pond water is normally beneficial, there are considerations to keep in mind. These are either associated with foreign elements to which your pond may be exposed, possibly without your knowledge, or uncontrolled nutrient build-up as a result of improper filtration and aeration.
Sometimes, pond water parameters, particularly pH levels, may also be incompatible with some plants. It may be best to steer away from watering plants with pond water if you suspect that it contains the following:
1) Overabundance of nutrients
Toxic levels of ammonia and nitrites, along with flare-ups of specific macro and micronutrients, may harm your plants. These tend to occur as a result of improper pond maintenance, and should not be a cause for concern if your pond system is healthy. Drastic increases in nutrients may also be a result of overstocking your pond with fish or forgetting to remove decaying foliage. Simply be conservative when stocking your pond – remember that its carrying capacity is limited by its volume – and regularly observe your fish communities for irregular behavior.
Rainwater or excess water that drains into your pond from the surrounding environment can also be a source of nutrients, particularly if it runs over-fertilized soil. If your pond acts as a drain for your garden, it may be wise to test for nutrient concentrations, not only to ensure that the water can be used for plants, but also to prevent any fish fatalities!
2) Presence of pathogenic microbes
Unfortunately, it is not always possible to control which microbes enter our ponds. Without our knowledge, there are many vectors that can introduce these. For example, bacteria from bird droppings or wild animals that previously visited toxic environments can carry fungal infections, destructive parasites, or viral loads that may proliferate in water and harm your plants. Poor aeration can also induce the growth of harmful nonaerobic bacteria.
The presence of these microbes may be indicated by poor fish and aquatic plant health, especially when all other parameters are maintained. If you suspect that your pond contains pathogenic microbes, it may be best to use another freshwater source for your plants. You may need to utilize more biosecurity methods to prevent the entry of these harmful pathogens, and make sure that you purchase your fish and pond plants from reputable shops.
Greywater refers to runoff or wastewater that could potentially contain harmful substances. A pond that collects wastewater can simply be the result of medieval plumbing from a nearby residence or something as serious as improper effluent disposal from factories. These will typically be quite diluted in case they do drain into your pond. To evaluate whether or not your pond is exposed to greywater, look into the history of your property, the surrounding establishments, and the stability of nearby sewage tanks and pipelines.
Final Thoughts – Is Pond Water Good for Plants?
If you have a healthy pond community, odds are your pond water is completely safe for your plants! Pond fish and aquatic plants are great detectors of harmful or toxic conditions. Prior to any fatalities, fish will typically display behavioral changes that signal the presence of pathogens or imbalanced water chemistry.
Similarly, aquatic plants may have difficulty competing with algae or may develop stunted growths. If none of these occur and all is well, you can confidently reuse your pond water. Even edible herbs and vegetables may benefit from the added nutrients. Do be more critical and observant, however, if you intend to use pond water for plants that you intend to consume raw.
Bear in mind that plants are sensitive to pH levels, so it is advisable to check if your pond water’s pH is compatible with their requirements. You may even opt to submit samples to a professional lsaboratory for peace of mind. They should be able to check for nutrient imbalances and several harmful pathogens. At the end of the day, if you’re still anxious about the effects of using pond water, you can always test it on a few plants prior to watering your whole garden. You’ll likely find that your plants will love the bonus nutrients!