Hydrangea cold question?

williamrJanuary 19, 2009

I've never posted in this forum before but I have decided to try some hydrangeas here in the Orlando area after seeing some beautiful ones at Lowes. My concern has been about them getting enough chill in the winter to do well here, so the fact that we are having some chilly weather should be good news for them. However, we are supposed to freeze a couple nights this week (28F-31F) and I am wondering how much protection actively growing hydrangeas need from a light freeze? Thanks everybody.


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This year, you could try covering them in order to protect the flower buds that have already developed near the end of the stems. If the plants are on containers, you could temporarily bring them inside too.

Next year, the plants will be established so I would not take action unless the last few days/weeks were hot and you were going to get a sudden dip below freezing.

Temperature swings up and down are the ones which can cause buds to get zapped so be wary of times when the weather warms up and then crashes below 32F. Back in 2005 or 2006, we were in the 70s daily until mid-December and then got freezing weather for a w-h-o-l-e week. The total number of blooms in Spring was... two.

So basically, a dip into the upper 20s will not kill the plants. At worst, some buds may get zapped, some leaves may turn ugly and some stems could get killed but the plants should survive and come back. Besides, your plants need to go dormant.

During the first year, you want the plants to get established in your garden so do not worry much about bloomage. Try to maintain the soil moist (not wet, not dry) most of the time. Add 3-4" of any type of acidic mulch up to the drip line. And make sure the location does not give the plants too much sun in the summer or the leaves will burn (choose a spot that gets shade starting at 11am or 12pm... during the hot summer months).

    Bookmark   January 19, 2009 at 11:44PM
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Thank you Luis! I think the location I chose is perfect since it is mostly shaded in the summer, with a bit of morning sun. Since they are still in pots I brought them into the carport and put sheets over them since I don't want to keep them warm, just above freezing. What type of mulch specifically is acidic? THanks again.

    Bookmark   January 20, 2009 at 5:56PM
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You are welcome, Michael. Pine products are acidic. Pine needles, for example, are acidic and a nice form of mulch. For mulch that is sold in bags, you need to read the name of the product (look for the words acidic or pine) or better yet, ask nursery personnel.

Mulch with a name like shredded pine mulch, you would want. Mulch with a name like pine nuggets would be acidic but is not recommended as it tends to float away with rain and wind. Mulch made of hemlock would also be acidic but people with roses should not try it because it contains blackspot fungus.

Like I suggested, when you go into a nursery, ask the people there to show you where their bags of their acidic mulch are.

    Bookmark   January 21, 2009 at 12:09AM
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Oh dear......I think we need to clarify a few things about mulches:-)

First (and the biggie!!) is that acidic type mulches (pine or cedar bark, pine straw, oak leaves, etc.) DO NOT affect soil pH. This is a very persistent gardening myth that has no basis in science. But it is a case of confusing association with causation. Trees that are considered to be "acidic" in their preferences do not cause the soil to become acidic but tend to grow where the soil is naturally acidic. Acid soil is created by its mineral base and by the amount of rainfall - areas with heavy annual rainfalls tend towards acid soils; dry, arid areas tend to have more alkaline soils. And both pines and oaks as well as a number of other conifers (typically considered as 'acidic' or 'acid producing' plants) can grow very happily in more alkaline soils - many species prefer it.

These types of mulches do not leach acid into the soil to any significant degree - certainly not enough to alter pH. And as with all organic matter, they approach neutral as they breakdown and decompose.

And there is no relationship between hemlock mulch and rose blackspot. Blackspot, Diplocarpon rosae, is unique only to roses - there are no other host or carrier plants. If any mulch carries it, it is because the mulch was collected where infected roses are grown and the spores came along for the ride. Hardly likely the case with hemlock or any other common wood mulch - roses don't generally grow in the forests or in lumber mills where the bark is processed.

If you need to acidify your soil, sulfur is the only recognized soil amendment that will have any significant effect, also peat but to a much less extent. Mulches will have no effect at all (don't use peat as a mulch - it dries out too easily and then becomes hydrophobic). And the effect of any acidifying amendment will not be permanent - virtually all soils have a natural buffering capacity that will return them to their original pH. You will have to keep reapplying periodically.

As to mulches - use anything you like and that is easy/inexpensive to obtain. What you are doing primarily with mulches is shielding the soil to moderate temperature swings and to reduce moisture evaporation. And keeping weeds to a minimum :-)

    Bookmark   January 21, 2009 at 12:30PM
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No, gardengal48, you misunderstood the conversation. We were not discussing ways to acidify the soil. We were just talking about examples of acidic mulches to use with the hydrangeas. As for hemlock, I have heard of this problem by word of mouth mostly when people use hemlock leaves (not shredded bark) as mulch so I am just passing the info. I do not grow the tree here and my information source (relatives) are in the NE (Mass.). I could not remember where I also read of that once but you got me curious. I found what I call "a commercial anecdote", comments from a nursery where no scientific basis is given: http://www.ilg-mv.com/Pages/mulch.html What have you heard about this type of comment? I would be very interested in knowing if someone has found hemlock leaves to be a problem or not. My rose books do not mention a connection between the two; only people (or in this case, the nursery) have.

    Bookmark   January 22, 2009 at 1:20PM
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Luis, I totally understand what you were trying to say.....you recommended using an 'acidic' mulch. What I was attempting to point out (and apparently failing :-)) was that it doesn't make any difference if the mulch is considered 'acidic' or not. It will provide no more benefit to the hydrangeas than any other type of organic, non-acidic mulch. Mulches comprised of products that we generally consider to be acidic in nature - pine straw, pine bark or shavings, other conifer mulches or oak leaves, which do indeed test out acidic on their own - do not leach that acidity in any measurable degree into the soil. In this case, a mulch is a mulch is a mulch :-)

As to hemlock mulch and rose blackspot, I have no idea why anyone would have made such a statement. There is no relationship between the two and anyone who has has studied even basic plant disease pathology would be aware of that. Even home gardeners whio research blackspot for control purposes in their own gardens will rapidly arrive at that conclusion. While the Internet is a wonderful source of info about a vast array of subjects, it also offers a lot of misinformation as well. Sometimes it is hard to tell what's accurate information and what is not. But I think in this case, simple logic would tell you this is not a valid claim.

    Bookmark   January 22, 2009 at 6:02PM
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Actually, I recommended the acidic mulch for another reason, cost. The pine mulch is local and cheaper that may not be the case everywhere though) so that is what I only buy. But, indeed, mulch is mulch is mulch is....

I asked my relatives about the hemlock and blackspot stories last weekend and, well, I was again told not to mix the two, that so-and-so had the same problem and etc etc etc. Well... let's just say that I let things be and changed the subject. Hee hee hee!

    Bookmark   January 27, 2009 at 2:58AM
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