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Pond Temperatures:
The Answers to Many of Your
"When Should I....." Questions

Some of the most common types of questions we're asked have answers that are specifically related to the temperature of the water in your pond. Questions like:

  • When should I start feeding my fish and turn on the pump in the spring?

  • When should I clean my pond?

  • When should I start to add bacteria to my pond?

  • When should I repot my pond plants?

  • When should I add tropical plants to my water garden?

  • When should I stop feeding my fish and turn off the pump in the autumn?

  • As pond temperatures will vary depending on the local weather conditions and your specific micro-climate, the following information should only be used as a rough guide. Please note that the temperatures we mention are Fahrenheit degrees and refer to the water temperature, NOT to the air temperature. They're also approximate temperatures as you'll see some variations listed in the literature related to water gardening.

    As spring air temperatures begin to rise, it's easy to be fooled into thinking that it's now safe to start tinkering with your pond. This can be a tricky time of the year though and you really need to pay attention to the weather forecast and be aware of any potential cold spells that are due to arrive. This is especially important if you've already started running your pumps and feeding your fish.

    Between ice-out and 40 degrees you can begin to clean the accumulated debris out of the bottom of the pond, as well as clean any filtration systems that you might have. If you haven't already been using them throughout the winter, you can also begin adding cold-tolerant brands of beneficial bacteria (e.g., MicrobeLift Autumn/Winter, Exotic Waterscapes Cold Temperature Beneficial Bacteria) to 'seed' and jumpstart your filter system. You can now also put in and turn on your pumps to encourage the beneficial bacteria to re-establish in any filtration system that you might have.

    It's important to note that the beneficial bacteria that 'coat' your filter material need the oxygen that's created by constantly moving water to survive. As soon as you turn off the pump, these oh-so-important algae-fighting bacteria begin to die and then the balancing process has to start all over to prevent the pond from greening up. That being said, once you've added beneficial bacteria products to your pond, it becomes critical to leave your pump running during the main water gardening season.

    Around the same time (near 40 degrees), you can begin to move your existing plant material from the pond bottom (or from wherever you stored them) to their proper place in the pond. You can now also divide, repot, and fertilize your aquatic plants to provide them with the longest growing season. Hold off on fertilizing your plants again until the water temperature reaches approximately 60 degrees. At that point, you can start to fertilize them at the rate of once a month. When the pond temperature rises above 75 degrees, you should fertilize your aquatic plants more often, such as twice a month.

    When it comes to introducing new aquatic plants to your water garden, there are also certain water temperatures in the spring and summer when it's best to do so. Around 35 degrees you can begin adding hardier marginal plants such as iris and marsh marigolds. At 50 degrees you can add dormant (i.e., those that haven't yet put out new growth for the season) hardy water lilies and dormant hardy marginals. New hardy oxygenators can be introduced when the water nears 55 degrees. At 65 degrees you can add tropical water hyacinth, water lettuce, and actively-growing hardy marginals. New tropical marginals and tropical water lilies should be added ONLY when the water temperature reaches 70 degrees.

    As for your fish, there are a number of natural existing food sources already in your pond even when it's cold (think algae, decaying plant material, bugs, frog and fish eggs, etc), so there's seldom really a need to feed them even if they do look like they're begging (large fish populations WILL, however, likely need additional food added to the system). If you do decide to feed your fish - as most pond owners do - you can start to feed them low protein food in the spring but only once the water temperature begins to stay at least 50 degrees or warmer. As silly as it might sound, some people rely on plain Cheerios as an easily digestible food source for their fish in the spring. Whatever you decide to feed them, DON'T give them large quantities and be very careful of feeding them if a cold spell is going to be a problem for a couple of days. The metabolism of your fish will begin to slow the colder it gets and having undigested food in their system during that time will make them more susceptible to disease and illness.

    Once the water temperature nears 65 degrees, you can switch to using a high protein fish food. If your pond ever gets above 85 degrees, you should check the daily dissolved oxygen level of the water as the health of your fish could suffer at such high temperatures.

    As your pond begins to cool down in the autumn, 60 degrees is when you can switch back to feeding your fish low protein food. Around 45 degrees you should completely stop feeding the fish.

    Once the pond water reaches 40 degrees and before it reaches the critical freezing point of 32 degrees, you should once again move your plants to the bottom of the pond (or wherever you plan on storing them for the winter) to prevent them from freezing. Remove any dead or decaying plant material from the pond, as well as any remaining water hyacinths and water lettuce as this will help reduce the amount of toxic gasses that can build up under the ice. You might even want to temporarily place a net over your pond to further reduce the amount of leaf litter that will enter your pond during the autumn. Be sure to remove the net before the snow begins to fall. During the same time period (before the pond freezes over), you can also remove and winterize your pump.

    Some people actually leave their pump running all year long. HOWEVER, it's worth noting that leaving your pond pump(s) running in cold weather can be a VERY risky situation. Not only is there a risk of creating 'ice dams' and having a catastrophic loss of water, but there's also the risk of 'super-chilling' your pond and severely stressing your plants and fish. Daily checks for ice dams and keeping the pump located near the surface of the pond (where the colder water will be) can help prevent some of these problems.

    As the pond begins to freeze over in the winter, you should use something like a pond de-icer or waterproof aeration kit to keep an area free of ice so that the accumulation of gases, which are toxic to fish, can escape. Obviously, if you don't have any fish in your pond, this won't be a critical factor.

    Now that you know how important water temperature is in the maintenance of your water garden and its' inhabitants, perhaps one of the cheapest - but most valuable - tools you can have on hand is a pond thermometer. Leaving one suspended in the water column (preferably at least 18 inches below the surface) will provide you with a quick reference point to help you make educated decisions about how best to tend to your pond. And please always remember - it's NOT how we feel about the current air temperature that dictates such decisions. A quick dip of your fingers or toes in the water might just change your mind about it finally being warm enough to start tinkering!

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