Hydrangeas bloom blue (instead of pink) when grown in acidic soil. Sometimes the hydrangea will change color over time as soil acidity levels vary. Therefore, if you want to maintain the blue color of the flowers, you will need to monitor the soil's pH level and take the necessary steps to preserve its acidity. This can be done relatively easily: just follow the steps below to get started.
Steps
Part 1 of 3: Understand the Science
Step 1. Know that blue hydrangeas grow in acidic soil and pink hydrangeas grow in alkaline soil
Hydrangeas are very unique garden plants due to their ability to change color according to the pH level of the soil. This means that the color of the hydrangeas in your garden will depend on the acidity or alkalinity of the soil.
- The scientific reason behind the phenomenon is that different levels of aluminum become available to the plant depending on the soil's acidity level (also known as pH). Acidic soils have a high aluminum content, which gives the blue color.
- An alkaline soil will produce pink flowers on hydrangeas; an acidic soil will cause the same plant to produce blue flowers. The exception to this are white or green hydrangeas, which are original varieties and do not change color. You won't have any success trying to change the color of the white hydrangea to pink or blue.
Step 2. Test your soil pH level
To see if the soil in your garden is alkaline or acidic, you will need to test the pH levels. This will help you predict the likelihood of growing blue hydrangeas.
- Soil with a pH of less than 5.5 will result in bright blue flowering hydrangeas.
- When the pH is 5, 5 to 6, 5, the flowers will be a different purple color.
- Soil with a pH greater than 6.5 will make the flowers pink.
Step 3. Use white vinegar to assess whether the soil is alkaline or acidic
You can get a good indication that your soil is alkaline or acidic by using white distilled vinegar. Just take a handful of your soil, put the vinegar on top and wait for the result.
- If the vinegar boils and bubbles when it comes into contact with the soil, it means the soil is alkaline and will produce pink hydrangeas. The more intense the effervescence, the more alkaline the earth.
- If nothing happens when the vinegar comes in contact with the soil, it means the soil is neutral or acidic and is more likely to produce blue hydrangeas.
Step 4. Test your soil's pH scientifically
If you want to know the exact value of your soil pH, you can test it using a home pH test kit. These are widely available at garden centers or online, just follow the instructions on the package.
Alternatively, it is possible to send a sample of your soil to a local garden center where they will test the pH for you
Part 2 of 3: Make the soil more acidic
Step 1. Sprinkle elemental sulfur over the soil
To keep your hydrangeas blue in color, elemental sulfur should be sprinkled on the soil around the bush to lower the pH to below 5.5. The exact amount of elemental sulfur will vary depending on the type of soil and how much the pH is. needs to be adjusted.
- Loam or clayey soil will require 350 g of elemental sulfur to lower the soil pH by an area of 2.3 square meters in one unit. In other words, it will take 350 g to lower the pH from 6 to 5. Sandy soil or sandy soil, on the other hand, requires less than 115 g of elemental sulfur to lower the pH by one unit.
- Start spraying elemental sulfur approximately 60 cm from the line of the outer edge of the shrub. Spread evenly over the soil about 10 to 15 cm away from the stem. This is the area where most of the roots grow and absorb water and nutrients.
- Use a small hand rake to mix the elemental sulfur into the top of the soil, then generously water the area to aid absorption. Elemental sulfur will likely have to be reapplied over time to maintain the blue color of the hydrangea.
Step 2. Use acid compound and powdered aluminum sulfate
To plant hydrangeas in a garden with alkaline soil and get a blue flower, you need to incorporate a sufficient amount of acidic compound and aluminum sulfate into the soil. Continue with regular applications throughout the life of the plant.
- You can buy acidic compost at the garden center - it is usually labeled "Ericaceous". Aluminum sulfate is available as a powder at garden stores or online (it is often labeled "blue hydrangea powder"). Be careful not to apply the powder directly to the root of a plant as it will burn the tissue.
- Instead, add a tablespoon of aluminum sulfate to 4 liters of water and use the solution to water ripe hydrangeas throughout the growing season. Don't be tempted to apply stronger concentration as this can burn the roots.
Step 3. Apply a fertilizer that is low in phosphorus and high in potassium
All hydrangeas benefit from good fertilization. To produce or maintain blue flowers on a shrub, use a fertilizer that is low in phosphorus and high in potassium.
- This can be labeled as a suitable fertilizer for azaleas, camellias and rhododendrons.
- Avoid using fertilizers like bone meal, as it will make the soil more alkaline, undoing all the work you've done.
Step 4. Use organic matter to increase soil acidity
If you don't like using chemicals in your garden, an application of certain organic materials such as grass clippings, leftover fruit and vegetables or used coffee powder can make the soil slightly acidic.
- The coffee powder used is very effective, just make sure it has cooled completely before incorporating it into the soil around the base of your plant.
- You can also incorporate ground coffee into the ground when planting a new hydrangea bush. See if your local coffee shop can provide some.
- Keep in mind that organic matter changes soil acidity more slowly than chemical powders and fertilizers. So, you will have to be patient if you decide to go that route.
Step 5. Water your hydrangeas with rainwater
Try using rainwater (instead of tap water) to water your hydrangeas. If you use heavy drinking water in your blue hydrangeas, this will neutralize the soil's acidity and the flowers will turn pink. For information on how to collect rainwater, see the appropriate article on wikiHow.
Step 6. Consider growing your hydrangeas in a separate pot
Rather than trying to change the acidity of the soil in your garden, it may be easier to simply plant new shrubs in a pot containing acidic soil.
To get blue flowers immediately, water the vase with an aluminum sulfate solution as described above
Part 3 of 3: Know what to avoid
Step 1. Avoid planting hydrangeas in chalky soil
Changing the pH of your soil will only work if you have lime-free soil. It is possible to identify if it has limestone soil if you notice the presence of lumps of chalk or stone in the soil. You will notice that the water drains easily and does not form puddles. The terrain will also be very dry in summer as the water runs off without being absorbed.
If you come across a limestone soil, do not try to change the pH of the soil, as it will not work. It is recommended that you grow hydrangeas in pots or containers with acidic compost instead of using soil from your garden
Step 2. Do not plant hydrangeas near any concrete structures
Concrete from a structure close to the flower bed (such as a concrete base or path) will affect the pH of the soil. Concrete will affect the soil and make it alkaline, which will obviously make it difficult to grow blue hydrangeas. If possible, plant hydrangeas away from concrete structures to maintain the blue color.
Tips
- If you change your mind and want to go back to pink hydrangeas, you will need to incorporate dolomite limestone into the soil or use a fertilizer with high levels of phosphorus. Expect to do this regularly throughout the life of the plant.
- If your hydrangea can't decide what color it will be (a mixture of pink and blue) or it's purple, then your garden's soil is likely to be quite neutral. To make the color bluer, apply aluminum sulfate as described above at a concentration of 1 tablespoon to 4 liters of water. The color will gradually change to blue.
