How to Get the Best Hydrangea Fertilizer

Spring mulch

When spring is around the corner it’s time to consider feeding and mulching our hydrangeas. An early feed will encourage rich colourful blooms, and if the time is taken to adjust the pH you can influence the colour of your blossoms over time. Mulching will help reduce your watering and weeding will also protect the roots of your shrub from compacting spring rains. Organic mulch feeds should be applied after the last frost to a maximum depth of 5 cm (2 in.) just under the shrub’s dripline. 

The natural pH of your soil

Start by knowing the natural pH of your soil. On the West Coast, the soils are acidic, but in the drier Interior, they might be alkaline. An inexpensive pH test kit will give you a head start.  

For blue flowers, the soil pH should be 5.2 to 5.5. If yours is higher, add 1 Tbsp. aluminum sulphate per gallon of water throughout the growing season. The aluminum is part of the blue pigment molecule. Avoid fertilizers with high levels of phosphates, as they have the opposite effect. 

For pink flowers, the soil pH should be 6.0 to 6.2. To get to this pH, add dolomite lime three or four times a year. Use a fertilizer with high levels of phosphorus, which keeps the aluminum from entering the plant’s roots.  

Changing the pH of your soil

Changing the pH can take a few years and won’t affect the flower colour right away. Growing a hydrangea in a large pot would give you more control over the pH. Since they are hardy to zone 6, they can be safely left outdoors in a pot all winter in zone 8. Otherwise, winter protection should be provided.  

Blue Booster

1 bag fish compost  

500 mL (2 cups) organic fertilizer (be low in phosphorus!) 

250 mL (1 cup) glacial rock dust (which naturally contains aluminum sulfate) 

5 litres (20 cups) fine leaf mould or home compost 

Pink Pick me Up

1 bag mushroom manure  

500 mL (2 cups) organic fertilizer 

500 mL (2 cups) lime 

250 mL (1 cup) glacial rock dust 

5 litres (20 cups) fine leaf mould or home compost 