Friday, February 4, 2011

Blue Hydrangeas

Hydrangea macrophylla -- Hortensia or Florist Hydrangea

Here in southern Ontario Blue Hydrangeas are quite rare. This is mainly because our soil pH tends to be neutral to slightly alkaline. Blue Hydrangeas require an acidic soil with a pH of 5.0 to 5.5. A secondary reason for the scarcity of this plant is due to our often harsh winters. As a general rule, plants in this species flower only from the end buds of upright or lateral shoots produced during late summer and fall of the previous season. A harsh winter can easily kill these preformed buds.

For those of you who love a good challenge there are several things you can do to increase your chances of success when it comes to growing blue Big Leaf Hydrangeas.

  1. Test your soil to see what your current soil pH level is. This can be done using a simple home testing kit which can be purchased at many garden centers or through
  2. Adjust your soils pH: To increase the acidity of your soil add a soil acidifier like agricultural sulphur or pine needles. Aluminum sulphate can also be used; it lowers the soils pH as well as supplies the plant with aluminum which is necessary for the blue colouring. (Note: as the pH drops to 5.0 to 5.5, aluminum becomes more soluble.) It can take years to drastically change the PH level of a soil. You will need to regularly test and amend your soil if its natural PH is quite different from the one you are attempting to create.
  3. Wait at least 1 month to fertile, especially fertilizers containing phosphorous (as phosphates precipitate aluminum).
  4. Prune your Hydrangea macrophylla as soon as the flowers have faded and strong shoots are developing from the lower parts of the stems and crown. Remove at the base some of the weaker shoots that are both old and new. Always try to keep several stems of old productive wood, with a sufficient number of stout new stems that will flower the following season.
  5. Winter protection: in December tie the shoots together and wrap with burlap 'or' for the really sensitive florist types place a screen around the plant before the temperature falls below -4 degrees C. Fill this with an insulating material such as coarse peat moss, vermiculite or bark. More material may be added if it settles during the winter. Remove this protection when the crocuses flower but protect on cold nights. If you are lucky, the flower buds will survive.
Photo credit: Blue Magic Green Houses

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