Changing the colour of your hydrangeas

By Melanie Mathieson
Gardening Guru

The last column talked about the new varieties of hydrangeas that will grow in our zone. I am sure that you can hardly wait to run out and buy one of these exciting new plants. This column will show you that hunting one down is just half the fun.
The Endless Summer “Original” Hydrangea blooms in two colours blue and pink. It doesn’t really matter which colour you buy because the colour of the flowers is determined by the pH of your soil. Alkaline soils (pH 6-7) will produce pink blooms. More acidic soil (pH 5-5.8) will turn your blooms blue. If you want to find out the pH of your soil, you can buy a soil pH testing kit at any garden supply centre. Now it is possible to manipulate the color of hydrangeas, but one word of caution: many people have killed their plants by applying too much product. As I always warn more is not better.
If you start with a blue flowered plant and your soil is alkaline, once planted the blooms will slowly convert over to pink and vice versa. So if you want a specific colour in your garden and your soil is not the right pH to maintain the soil you want you can manipulate the soil. You may consider growing your hydrangea in a large pot where it would be easier to control the pH. You can also buy a large pot and bury it a ground level and then plant the hydrangea in the pot. This will allow you to keep your soil treatment contained.
To change from blue to pink or maintain pink blossoms you need to change from an acid soil to an alkaline one. To help raise your pH you can use dolomitic lime several times a year.. Use fertilizers with high levels of phosphorus such as 10/25/10. Phosphorus helps to prevent aluminium from being taken up in the plant’s system causing the blossoms to turn and remain pink. After treatment you will want to test your soil and aim for a pH of about 6.0 to 6.2. If the pH rises above 6.4, your hydrangea may experience an iron deficiency and suffer growth problems.
Using a soil acidifier will to lower your soil’s pH to a level of about 5.2-5.5 will produce beautiful blue flowers. You can acidify your soil with a chemical process or a compost process. If you are going to use chemicals you may want to start by using a fertilizer that is meant for evergreens, azaleas or rhododendrons. These are products like Miracid and elemental sulphur or iron sulphate. Sulphur works slowly while iron works more quickly but requires more ongoing applications. You must apply these products at the recommended rates so they will be safe for your plants. You can also buy products called acidifiers. Companies prepare mixtures of sulphur, iron, and other chemicals specifically for reducing the pH of garden soils. They are effective but can be expensive so they are best used for small areas. Check in any garden supply retailer in our District for these products. Make sure you follow the directions very carefully when using any of these products and you thoroughly water the plant and surrounding soil before adding any acidifier. Another method is to add large amounts of organic matter, such as peat moss, composted leaves (especially oak) and evergreen needles that are acidic in nature and lower the pH as they break down into the soil. Remember to check the pH of your water as well as your soil. Hard water contains lime, which raises pH levels. The higher the pH level, the harder it becomes to attain blue blooms. Also remember that concrete foundations, walkways and paving stone driveways tend to leach lime, raising the pH in that area.
Remember when applying any of these pH altering methods that you are just trying to affect the soil in close proximity to your hydrangea not the whole garden. Be careful that you keep the applications close to the hydrangea to avoid killing any of the other plants. One word of caution though, some gardeners say they use vinegar to change the pH of their soil. I do not recommend this. Yes, vinegar is an acid but it is a different acid than the products I recommended to use. Play it safe and be kind to your soil and plants and use what is recommended in this column instead of some home brew or old timer’s recipe. There is no need to alter the colour of your Hydrangeas if you are happy with the results that with your soil produces in its natural state. If this is the case care for your hydrangea as you treat the rest of your garden in the general sense for fertilizing and watering.
Fortunately the Endless Summer hydrangea varieties are is very forgiving and will not suffer if left unpruned or pruned at the wrong time. In fact, young, recently planted shrubs are best left alone. Unlike other hydrangeas, the Endless Summer will bloom on both old and new wood, branches that grew last year and the new branches from this year. Another unique feature is that this hydrangea will continue to set buds and bloom throughout the season. Make sure to deadhead the spent flowers to encourage new blossoms. Feel free to cut the blooms for drying or to display fresh in vases because this type of pruning actually encourages the plant to produce more flowers. Many people like to leave the spent blooms on their plant because it adds winter interest and theory has it, it may also act to insulate the new buds from frost and cold. The spent blooms should be removed in spring however. Spring is the best time to prune. Once you see new growth you can prune back the old branches to a finger width above the new green growth.
An Endless Summer hydrangea can used in many ways and is ideal for every area of your garden and home. You can introduce them to your landscape as specimen plants, Foundation plantings, shrub or perennial borders, in container gardens as patio or deck accents and do not forget that dried flowers make gorgeous wreaths, floral arrangements, and vase displays.