Type “baking soda garden” into your web browser and you get over a million hits. Not surprisingly, most are something along the lines of “17 Smart Baking Soda Tips,” and “7 Natural Uses for Baking Soda.” Depending on which list you read, it sweetens tomatoes, increases the blooms on geraniums, begonias, and hydrangeas, prevents black spot on roses, cures powdery mildew, discourages soil gnats, and kills slugs and other harmful insects “while not harming beneficial insects.” (So tell me—how does it know which are the bad bugs?) Plus, it’s natural, cheap, and readily available.
These are some pretty impressive claims, and if you’ve been reading my blog for any time, you know that I tend to be skeptical of anything that claims to solve all your problems, gardening or otherwise. Is using baking soda in the garden a good idea? Do these tips and remedies actually work? Is there any research on the subject?
It turns out that yes, there is a lot of research, and the results are pretty clear. Baking soda (aka sodium bicarbonate) does help to prevent fungal diseases. This is because it is alkaline, and fungus spores prefer acidic conditions. However, once the fungus has taken hold on the leaves, baking soda will not kill it. So technically, it’s “fungistatic” (it wards off fungal diseases), not fungicidal.
But don’t run out and buy a big box of baking soda to put in your garden shed. There’s more to this story. The studies that support the use of baking soda as an effective preventative were largely done in controlled, laboratory conditions. When used in a greenhouse or outside in the garden, it’s much less effective.
Baking soda is usually mixed with water and sprayed onto the leaves. But too much can cause discoloration, chlorosis, and burning. A solution as weak as 1% can damage your plants. At the same time, for the spray to be effective, higher concentrations are better. There’s a fine line between preventing fungal diseases and killing your plants.
If you do manage to find the right balance, you next face the challenge of applying the spray so it sticks to the stems and undersides of the leaves. And if you finally achieve good coverage, the next rain storm will wash it all off again. Those fungal spores haven’t gone anywhere, however, and as soon as the pH dips, they’ll germinate.
If you are constantly fighting fungal diseases in your garden, perhaps you need to rethink your landscape. The best way to reduce or eliminate diseases is to match the right plant with the right place. if a fungus does attack, there are other products available that actually work. Powdery mildew is the primary culprit here in Colorado; you can read my post about it here.
How about the rest of the claims? There is no basis for the claim that baking soda sweetens tomatoes (“sweetening” the soil just means you’ve raised the pH—it has no direct impact on a crop’s flavor). Lime does a better job of raising the pH of acid soils, and you can control your application to get exactly the results you want. Dusting or spraying your plants with baking soda will hurt the plants far more than the bugs, although some claim that direct application of baking soda will eventually kill a cockroach. (I saw some impressive cockroaches in the tropics, but I didn’t have any baking soda handy so I couldn’t try this out.) Again, there are alternative ways of controlling garden pests that are both safer and more effective. Finally, I suppose a direct application of baking soda will kill a slug. I can’t try it out because I don’t have slugs in my garden. (And I don’t miss them a bit, either!)
Baking soda is a very useful product. We use it when cleaning, I’ve brushed my teeth with it, and of course it does a lovely job of making baked goods rise (when mixed with an acid). It’s used to clean aluminum panels, as a fire extinguisher, and to treat heartburn. You can entertain the kids by creating baking soda and vinegar “volcanoes.” Just don’t waste it in the garden.
Dr. Linda Chalker-Scott has written an excellent review of the literature on this topic. If you want something more technical, you can find her article here.