How to prune an old leggy mophead to promote fullness/new growth?

viche(7a MD)March 2, 2012

I have a few 20 year old Mophead (look almost identical to pics of Nikko Blue) hydrangeas that have gotten leggy and thin. They each have a few old stems that send out new shoot and flowers every year and a couple of new stems, but they are much less full then they used to be. What should I be doing pruning and fertilizing wise to get them back to being dense? I also wouldn't mind controlling their size, even if that means cutting them down to the ground and missing a year of flowering. I'm ready to be drastic here!

Details:

- They don't get a ton of sun. Mostly shade until late afternoon.

- They bloom at the end of June here in Maryland. (Will probably be earlier this year since tulips are already starting to come up and forsythia has started to bloom).

- I usually wait until early spring when all frost chances are gone, and buds are starting to leaf out. Then I prune each stem down to right above the healthiest buds...usually the 2nd or 3rd bud down.

- I've been afraid to cut too many of the really old stems down to the ground, even when they aren't producing many buds.

Thanks!

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luis_pr

Hello, viche. Well, normally the plant should generate new growth from the crown each year but being so old, I would rejuvenate the plant by cutting the one third oldest (or longest) stems down to the ground on year 1. Then the next 1/3 oldest on year 2. And then the rest on year 3. This type of pruning can be useful every 7-10 years.

I would make sure they get some fertilizer in April and July (in the southern half of the country). Or once in June (in the northern half). You can add coffee grounds and other weak fertilizers like liquid fish and liquid seaweed throughout the growing season. For fertilizers, you can use organic compost, composted manure, cottonseed meal or a general-purpose slow-release chemical fertilizer like Osmocote.

Lastly, consider increasing the amount of sunlight that they get (if you can do that, that is) to around 2 hours or so. I got the impression that they might be getting little sun from your wording and could not quite tell if reduced light levels might be causing reduced bloomage.

Luis

PS - I also recommend a soil test if you have not done one in the last 5 years or so.

    Bookmark   March 3, 2012 at 3:31AM
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viche(7a MD)

Thanks Luis. Sounds like a plan. It'll be hard to cut down those stems, but I'll just force myself to do it.
Which fertilizing plan would I follow in central MD? Does it matter that we have had a SUPER mild winter? It's been in the 50's and 60's many days this February. Maples are budding, forsythia blooming, lilies starting to emerge.
Yeah, I'll have to check on the sunlight. I thought it was around 2 hours.
I'm due for a soil test.
Do you think general pruning in early spring down to the strongest buds (2nd or 3rd from top) has been a good practice, or would it be better to prune before August and before new buds have even begun to develop? My practice allows me to selectively prune, doing it in August might allow the plant to put all of its energy into all the potential buds. Any thoughts?

    Bookmark   March 3, 2012 at 3:05PM
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luis_pr

You are basically on the edge of the "northern" and "southern" halves so feel free to experiment. If you fertilize in April & July, make sure that (a) fertilzing in April will not trigger growth that a frost will then kill and (b) fertilizing in July will not delay the start of dormancy in the Fall. You want the plants to start going dormant by or before your average date of first frost. Likewise, you want to fertilize after your average date of last frost in Spring. For example, for Baltimore, the date of last frost in Spring is around April 11 so, you could try fertilizing then or about 1-2 weeks afterwards. The average date of first frost in the Fall is October 29 so fertilize in July and observe if the plant starts going dormant by October 29 or 1-2 weeks earlier. Using organic fertilizers like organic compost, composted manure or cottonseed meal helps with this because they are slow-release fewrtilizers. Chemical fertilizers can also be used but look to see that they claim being "general-purpose" and "slow-release".

I would not prune these 20-yr old varieties in Spring because all mopheads, including the rebloomers, always develop their Spring blooms way back in July-August. So pruning before they leaf out in Spring basically cuts off the Spring blooms. With reblooming hydrangeas that is not an issue since they rebloom again later on. But your 20-yr old mopheads are probably not rebloomers. Best time will be to prune any time after they have bloomed in Spring thru and including the end of June.

    Bookmark   March 3, 2012 at 6:46PM
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gardengal48

I don't have any arguments with Luis' pruning advice but if these were my plants, I wouldn't have any issues about cutting back the entire shrub hard this year, understanding you are sacrifing a season of bloom. Such a drastic program on old, established shrubs seems to generate excellent results regarding the development of new, vigorous growth or at least it has in my experience.

I would also not necessarily wait until after last frost date to fertilize in spring. There is really no relationship between frost dates and fertilization and unless you are using a lot of fast acting synthetic ferts, nutrients won't be metabolized by the plant until later in the season anyway. I'd also consider using some alflafa meal or an orgfanic fert that includes alfalfa. Alfalfa contains a natural growth hormone that encourages stem development from the root crown, which is exactly what you want to do with rejuvenating caning shrubs like hydrangeas.

    Bookmark   March 13, 2012 at 6:22PM
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viche(7a MD)

I'll look for alfalfa fert. If I end up using Osmocote, should I wait until frost is done or can that be done earlier as well.

So this makes me a little nervous, but you are saying to cut every stem down to just about ground level? Should I do this now, or after the plant has sprouted some leaves and had a chance to collect some energy. I'm not a plant expert. Does the plant have enough energy to regrow from the ground if I prune it now? Is there any harm in enjoying this year's blooms and them pruning it after they fade?

Thanks

    Bookmark   March 13, 2012 at 6:31PM
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gardengal48

The plant's energy now is in the root system so a hard pruning when dormant will not set growth back. In fact, a hard pruning tends to revigorate the plant, especially if combined with proper fertilization. Whether you choose to do a hard prune now or later after bloom season is your choice.....just be aware you will likely miss a bloom season. If you prefer, the less radical 1/3 per year rejuvenation pruning will work equally as well but just takes longer.

As I said, there is no significant relationhsip between frost dates and fertilizing - i.e., fertilizing NOW will not encourage the plant to immediately start producing lots of tender new growth. Nutrient metabolization is not immediate and depending on source, can be a season long , or longer, process. And if you do choose the Osmocote, it is treated to be slow release so will not even begin to release any nutrients until soil temperatures are properly warm enough

    Bookmark   March 14, 2012 at 2:13PM
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viche(7a MD)

Will pruning now vs after bloom affect when i will see new blooms on the new stems, or will both result in only a 1 year (next year) of bloomlessness?

I see them prune hydrangeas so that they are about 10 inch stems here at work. You suggest to the ground?

    Bookmark   March 14, 2012 at 3:36PM
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gardengal48

Either timing should only result in a single season of bloomlessness at the most. I have personally experienced hard pruned bigleaf hydrangeas that DID produce flowers the same year as an early pruning but it is highly cultivar and climate related and not common, so don't want to get your hopes up. All this of course applies only to old wood bigleafs, not to any of the remontant or reblooming cultivars.

"to the ground" is a bit severe...4-6" is fine as is leaving them at 8-10" if you prefer. Depends on the size of the shrub you begin with. Do what feels good to you.

    Bookmark   March 14, 2012 at 3:54PM
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viche(7a MD)

Thanks so much!

    Bookmark   March 14, 2012 at 4:04PM
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