The Q&A Archives: Hydrangea

Question: I just bought a beautiful purple/blue hydrangea from the Botanical Gardens in St. Louis. I questioned if it was hardy in our region and was assured it was. However, it looks very much like one I bought elsewhere before and I believe it was some sort of florist variety for indoor use only. My question is--are all hydrangeas hardy for outdoor planting? How often should I fertilize it (when to start and when it stop)? Thanks.

Answer: There are many different kinds of Hydrangea, some are not very hardy and are used primarily as florist plants while others would be hardy into your zone. Your zip code places you in zone 6A or the coldest part of zone 6. Depending on your microclimate it might be as cold as zone 5. So it depends which cultivar you have, there are several cultivars of Hydrangea macrophylla and H. serrata that should grow for you. I would hope the Botanical Garden would know which ones would suit your area.

For best results you should plant it in a sheltered spot with protection from winter winds. If you have a variety that blooms only on older wood you may need to build a cage around it with a cylinder of wire mesh and fill that with nonpacking insulating material such as oak leaves or loose straw and top it with plastic to keep it dry inside. (Leave gaps in the plastic to allow for air circulation and prevent condensation.) You would apply this in late fall and remove in mid spring. Be prepared to cover the plant if spring frosts threaten after it begins to leaf out.

If you have a variety that blooms on new wood, you would have less to worry about. Simply mulch in late fall after it goes dormant and trim back any dead branches in the spring.

Routine care would include a top dressing with good quality compost each spring along with an application of general purpose or slow release granular fertilizer such as 10-10-10 per the label directions.

Your flowers should remain a good blue in acidic soil. If your soil is not naturally acidic, you could use a slow release fertilizer for acid loving plants such as Hollytone. Your local county extension should be able to help you test your soil to check the acidity and also fertility levels and advise you more precisely based on the soil test results.

Good luck with your hydrangea!

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