Baking Soda or Coffee Grounds for Blue Hydrangeas: Fact or Fiction? Many gardeners are fascinated by the chameleon-like qualities of hydrangeas, particularly their ability to shift bloom colors from pink to blue. This change is often attributed to soil pH, leading to a common question: can household remedies like baking soda or coffee grounds effectively alter soil pH for these beautiful shrubs? We asked our National Garden Bureau expert members for their take, and here’s what they had to say: Can Baking Soda Change Your Soil pH for Hydrangeas? Proven Winners Color Choice: “No.” “First of all, baking soda is a base, so even if it were effective, it would actually make your soil more alkaline. This is the opposite of what most people are trying to accomplish by changing their soil pH for hydrangeas, which is to turn bigleaf hydrangea ( H. macrophylla) from pink to purple/blue. For a hydrangea to undergo that color change, two things must be true: the soil must contain aluminum, which is what actually affects the change, and it must be acidic. Only in acidic conditions are the aluminum ions free from their bonds so the plant can take them up and metabolize them. Since adding baking soda would actually make the soil more alkaline – if it were effective in the first place, which it isn’t, at least not in any realistic quantity – it would prevent the hydrangea from turning blue. While it is true that in some areas, soil is naturally so acidic, or can become so acidic, that it is not favorable for plant growth, if you were trying to raise the pH of your soil, baking soda would not be the best way to do it from an efficacy or price standpoint. What’s the point of a short-lived soil pH change?” Southern Living® Plant Collection: “No.” “Baking soda (sodium bicarbonate) is a type of salt and is not recommended for use in the garden, as salts can accumulate in the soil over time. First, test your soil to determine the pH; acidic soils will result in blue blooms, while alkaline soils create pink blossoms. The most reliable and safest option for plants is to apply powdered or pelletized sulfur to the soil or use a fertilizer formulated for hydrangeas.” Bloomin’ Easy Plants: “No.” “Products designed to blue hydrangeas or acidify soil are more effective and better long-term. Aluminum sulfate is much more effective. Baking soda is alkaline and will do the opposite.” Lorraine Ballato, All Things Hydrangea: “No.” “Baking soda (sodium bicarbonate) is actually the active ingredient in many horticultural products labeled for control of powdery mildew. Like all treatments for fungal diseases, it doesn’t reverse the condition, but it does a pretty good job of arresting the pathogen’s progress when applied as a foliar spray. It essentially changes the pH of the foliage, which makes it unpleasant for the fungus. The pH of baking soda is pretty high (about 9-ish). That bit of info tells you that if you apply it to the soil, you have a high probability of raising the pH, which for hydrangeas isn’t good. A good pH for this genus is about 6. When you go lower, you can cause other nutrients to become ineffective and/or unavailable, plus the lower pH can change the color of the flowers in macrophyllas and serratas that are pH sensitive and spoil the health of the plant. Essentially, the science doesn’t support a positive outcome from a home-made use of baking soda for the garden.” Green Fuse® Botanicals: “No.” “There would be variabilities as it comes to soil condition (clay/loam/organic/sand) as well as water volume needed to disperse applications. So, without all the perfectly correct proportions it could easily kill a plant. It makes more sense to use the available products that have been created for this purpose.” Can coffee grounds effectively change the pH of your soil? Southern Living® Plant Collection: “No.” “Unused coffee grounds are highly acidic and aren’t generally recommended to use directly on plants in the garden. Spent coffee grounds have a neutral pH. The safest way to use coffee grounds in the garden is to add spent coffee grounds to your compost and use the compost around your plants once it has matured.” Lorraine Ballato, All Things Hydrangea: “No.” “Used coffee grounds do not change the pH of the soil enough to change hydrangea flower color. There may be a slight change, but as with all soil amendments, it takes time to get through the vascular system of the plant. On the other hand, fresh, unused grounds will burn the life out of the plant. NEVER use them!” Proven Winners Color Choice: “No.” “We hear this question quite a bit, so I will direct you to my go-to source, a Washington State Extension article, since I would just be rephrasing this anyway. The short answer is no, and that if you want to use coffee grounds in the garden, the best thing to do is to put them in your compost pile.” Bloomin’ Easy Plants: “No” “But I do add my used coffee grounds to my compost pile in the summer, and in the winter, I put them directly on my veggie garden and then dig them into the soil before planting.” Realistically, can you change the soil pH enough to impact the color of your hydrangea blooms using these “Homemade” methods? Bloomin’ Easy Plants: “No” “Products designed to blue hydrangeas or acidify soil are more effective and better long-term.” Proven Winners Color Choice: “No.” “If someone is trying to change the color of their hydrangea, we recommend the following: 1. First, you need to have a hydrangea that can change color in the first place. This means it must be one of three types: a big leaf, a mountain hydrangea, or Fairytrail Fresco Cascade Hydrangea. It also must be a pink/purple/blue variety, as white and green hydrangeas of these species will not change color. 2. Ideally, you should get a soil test, so you know whether your soil needs a pH change or aluminum or both, since the soil must be both acidic and contain aluminum to change the flower color. The majority of products on the market are merely soil acidifiers, so if the issue is that your soil doesn’t contain aluminum, such products would acidify your soil without ever changing the flower color. Aluminum is naturally occurring in many soils, but tends to be highest in clay soils over sandy ones. 3. You will always get the best results by using a product made for the purpose and following the package directions carefully.” Southern Living® Plant Collection: “No” “These methods may ultimately harm your plants and the health of the soil. There is no quick fix for changing the color of hydrangeas, and the effects of adjusting the soil pH may take months to produce a noticeable change; it often requires one or two growing seasons.” Does baking soda play any important and beneficial role in other areas of your garden? Lorraine Ballato, All Things Hydrangea Using baking soda for other garden needs has possibilities, but not enough to compensate for the potential adverse impacts. If economy is the driving factor in not buying a garden-approved bicarbonate treatment, it’s important to note that a very small jar is relatively cheap. Plus, you only use a very small amount of the product to get the right mix. Neem is just as good if not better. Copper-based products should be avoided as copper persists in the soil and is harmful to earthworms. Bloomin’ Easy Plants We don’t recommend using baking soda as pest control; it is easy to overdo it and do damage, so if you do want to try it, always test a bit first before you spray your whole garden. Baking soda can be used on garden tools and as a deodorizer for compost bins. If you want to help your tomatoes and peppers to produce well, you should be adding lime to the soil to add calcium and raise the pH a bit. Southern Living® Plant Collection Baking soda is not recommended for use on plants in the garden, as it is a type of salt and can be toxic to plants if overused. However, it can help remove rust on tools when mixed into a paste. Proven Winners Color Choice: Not really. While it has shown some promise in treating certain fungal diseases, it seems its strongest abilities are related to post-harvest processing and are more commercial in nature, according to this Washing State Extension article. From a home gardener perspective, it seems it is most effective if mixed with a horticultural oil or another spreader sticker according to this article from the University of Florida Extension. This obviously increases the cost and difficulty in applying such solutions so you’d be better off buying a soap or neem oil or some other product expressly created for the purpose. More info here from the University of Illinois that may be of interest. Though there are many claims out there that sprinkling baking soda around tomato plants results in sweeter fruit, no studies have been able to prove any effect. So the best way to change the color of a hydrangea is by: • Having a soil test to see the pH of your soil • Use proper ingredients that are made for this purpose.
Source: National Garden Bureau
