Koi are hardy fish, but the colder months are when they are most vulnerable. A little preparation in autumn sets them up to come through winter healthy and ready for spring. Here is how to care for your koi as the temperature drops.
How koi respond to the cold
Koi are cold blooded, so as the water cools their metabolism slows right down. Below around 10C they become sluggish and stop growing. As winter sets in they gather near the bottom of the pond, where the water is warmest and most stable, and enter a state close to dormancy, living off the reserves they built up over summer and autumn.
Feeding koi in autumn
The single most important thing is to feed by water temperature, not the date. A pond thermometer is your best guide:
- Above 15C: feed normally with a good quality koi food.
- 10C to 15C: reduce feeding and switch to a wheatgerm based food, which is gentler on the digestion in cooler water.
- Below 10C: feed only sparingly.
- Below around 6C to 8C: stop feeding for winter. Feeding koi when they cannot digest properly does more harm than good.
For the full feeding breakdown across the year, see our guide to feeding pond fish through the seasons.
Preparing the pond for winter
- Keep it clean. Net off falling leaves and remove debris. Rotting matter uses up oxygen and can release harmful gases, especially under ice.
- Cut back plants. Trim dying foliage from marginal and oxygenating plants before it collapses into the water.
- Check the depth. Koi need a deep area, ideally around 1.2m or more, that will not freeze solid, giving them a stable refuge through the coldest spells.
- Look after your kit. Service the pump and filter before winter so they are reliable when you need them.
Ice and oxygen
If your pond freezes over, never smash the ice. The shockwave travels through the water and can harm or even kill your fish, and breaking the surface does nothing to help. Instead, melt a hole gently by standing a pan of hot water on the ice, or use a pond heater or floating de-icer to keep an area open. That open patch lets harmful gases escape and oxygen get in, which is exactly what keeps koi safe underneath.
A quick autumn health check
Autumn is a good time to look your koi over while you can still see them clearly. Check for good body condition, clear eyes and skin, and normal behaviour. A koi going into winter in good health is far better placed to cope with it. If you spot anything of concern, deal with it before the cold sets in, as treating fish in very cold water is much harder.
Come spring
As the water warms again in spring, your koi will gradually become active and hungry. Reintroduce food slowly, starting with a wheatgerm based feed, and build back up as the temperature climbs. Get autumn and winter right, and your koi will be in fine form to enjoy the new season.
Feed your koi well into the cold
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