Greetings, Luis!

Thank you for your question to Ask Extension/ Ask a Master Gardener. You posed an excellent question and you stated it extremely well. Thank you for the detail!

I apologize that it has taken so long to respond; your question was escalated to insure you received a thorough response. I am a Master Gardener Volunteer (MGV) and an hydrangea enthusiast. In preparing an answer to your question, I referred to several research-based sites, and have provided the links to those below. I also included a couple of other sites that gave detail that might interest you. I vetted the information as much as possible.

Your first question asked at what temperature will macrophylla flower buds be damaged and/or killed.

Short answer: It depends whether or not the plant is dormant. During dormancy H. macrophylla , aka Big Leaf Hydrangea, can withstand temperatures to zero degrees Fahrenheit. Prior to and after break of dormancy, H. macrophylla can withstand temperatures above freezing, that is above 32 degrees F. The more severe the temperature and the longer the exposure, the greater the damage and eventual "death" of the buds. Desiccation by wind can contribute to damage. Protection from wind and cold in a sheltered location can mitigate damage.

Details and explanation:

- In the fall buds are being set on stems grown that year, later called “old wood”. Drops in temperatures below freezing can damage the leaves and the buds until the plant goes dormant. Keeping the plant well-watered until the first ground freeze, mulching late season and covering the plant if a sudden early freeze is expected will help protect the plant and buds from damage.

- Once the hydrangea macrophylla has entered dormancy, it can withstand temperatures to zero degrees Fahrenheit. (Some sources suggest negative ten degrees Fahrenheit.)

- “As the average temperature rises in the spring, the plant is signaled to begin growth of new buds on the stems. When this process is initiated, the plant is no-longer tolerant to temperatures below freezing.” (Quoted from Attack of the Killer Frost, cited below.) So, once uncovered, if a drop to below freezing is predicted, plants need to be covered in order to protect them from freezing,

Your second question was “Why not leave the winter protection until after the average date of last frost?” Good question!

- In the spring, as soon as temperatures rise to about fifty degrees F, plants begin their new growth. They need sunshine and air circulation to grow strong and healthy.

- Keeping plants covered as the weather warms above fifty can encourage the growth of pathogens (fungus, bacteria etc.) and attract pests (aphids, etc.).

Other notes

Interestingly, many hydrangea lovers in USDA Plant Hardiness Zones 6 and higher don’t cover their macrophylla after they are established (first few years) and instead let nature take its course. For instance, last spring, in Hamilton County, we had an early warm spell in the spring followed by a late freeze. Although my Endless Summer macrophylla are in a protected area, the stems and buds were frozen and there were very few blooms on the old wood. But this lovely and resilient plant produced strong new growth and bloomed prolifically later in the season. (I live in zone 6b. Most of Ohio is in zone 6, with some pockets of Ohio in zone 5b. Here is a link to help if you need to find your USDA zone. https://planthardiness.ars.usda.gov/pages/view-maps )

I hope I have answered your questions, but if my response generates further questions or if clarification is needed, you can respond to this email, and your response will come directly to me. Again, thank you for using our service.

-----------------------------------

Following are some articles you might find interesting and helpful.

Winter Care for Hydrangeas (from UNL) https://extension.unl.edu/statewide/buffalo/Yard/Winter%20Care%20for%20Hydrangeas%2012-24-2016.pdf

Growing Bigleaf Hydrangea (From University of Georgia Extension; cites reasons for not flowering, including use of fertilizer with high nitrogen levels.)

How to Protect Hydrangeas from a Late Frost (From SF Gate; has information about staking and mulching) https://homeguides.sfgate.com/protect-hydrangeas-late-frost-66049.html

How to Put Hydrangeas to Bed for the Winter (from Monrovia; great advice on mulching) https://www.monrovia.com/be-inspired/how-to-put-hydrangeas-to-bed-for-the-winter.html

Attack of the Killer Frost https://eldlandscape.com/attack-of-the-killer-frost-hydrangea-blooms-in-jeopardy/

Best of luck with your macrophylla hydrangea! Of all my hydrangea varieties, they are my favorites!