I recently decided to paint my translucent rain barrels because I was seeing a lot of brown slime build up on the inside of the barrel. Read more about that here. A recent class on produce safety in the home garden has gotten me thinking about the rain water from my rain barrels. First, I’ll say that I’ve used the rain water from the slimy barrel on my garden in the past without a care and have never had a problem. But think about it. Rain from the roof of the house going into the rain barrel contains dirt, pollen, possibly bird poop, decaying leaves from the gutter, insects, and maybe some of that black gravel like stuff that comes off the shingles. Then this mixture incubates in the warm sun until you are ready to use it on your beautiful garden. What could be wrong with that?
Maybe nothing. But maybe it’s good to start thinking about better practices in the home garden. I have come to regard rain barrel water as compost tea, i.e. compost mixed with water. I would use compost tea in a different manner than I would use potable water in the garden. Home compost doesn’t always get hot enough to kill weed seeds and pathogens. On the plus side, home compost doesn’t usually contain animal products or manure which would change how it is used in a food garden. (Side note: one year I did find a possum skeleton in my compost pile so you just never know) I know some gardeners use compost tea even as a foliar spray but I don’t recommend it. These are the questions to ask yourself when you decide whether to use rain barrel water (compost tea) or potable (drinkable) water in each situation.
Will the water touch the edible part of the plant? For instance are you spraying water overhead through a sprinkler or irrigating it on the ground. If you have tomatoes up off the ground, the water doesn’t touch the tomatoes so no worries. If you have carrots in the ground, the water does touch the edible portion. Are the edible parts low enough to the ground that water will splash up onto the edible portion during a strong rain? A thorough washing of produce is always in order anyway but don’t we sometimes pick a raw tomato, strawberry, or pod pea and eat it right there in the garden without washing it?
How hard is that produce to wash? Even if compost water touches the edible portion of the plant you can remove most contamination by thorough washing. Knobby carrots and crinkly greens and herbs can be more difficult to wash. According to experts at the university extension classes I’ve taken, plants cannot systemically take up pathogens, I’ve asked that more than once. However, a type of surface contamination called biofilm is very difficult to wash off. Here is an old but informative article on biofilm. Produce can also take up pathogens from water if the water is more than 10 degrees cooler than the produce. In the cooler water the produce flesh contracts drawing in the dirty water so rinsing under running water is better than a group dunking.
Will the produce be eaten raw or cooked? Obviously cooking kills pathogens that washing may have missed. For example, potatoes versus carrots.
Hopefully this information is more useful than scary. I am a fan of rain barrels. I will be using some exclusively for flower beds and will also use the rain water selectively in the garden. In addition, I’ll try to reduce contamination in the rain barrel by painting the outside (reduces light), rinsing them out each spring and waiting for a good rain to wash off the roof in the spring before I hook them back up.