In an interview with Dr. Michael Dirr, we asked "What are your recommendations for growing hydrangeas?"

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• Buy a reblooming one.

• Find partial shade. North or east side of the house is usually good.

• Moist soil if you can do it. Mulch them. Preserve moisture. Hydrangeas show terrific drought stress, 85 degrees and above they begin to show it. We’re looking for that trait too (in hybridizing), heat tolerance as well as drought tolerance. I don’t know if we’ll ever get that.

• When the flowers become brown and tatty, take them off.

• Do not cut them back in the spring until bud break.  This is the insane thing. People cannot help themselves. They want to prune them and clean them up way ahead of when they should. The flower blooms are forming. Even on the rebloomers, the flower buds form on old growth first. In the fall flower buds are all formed for next year. So don’t cut them back until bud break in the spring. Take a look at how much live tissue you have, which will be manifested in new leaves popping out at the nodes—there are two buds at a node. Prune the plant back to the green. Bonnie and I wait every year before we clean them up. Sometimes they’re killed back clean to the ground, the crown. They will come back, and if it’s a rebloomer, it will rebloom.

• You need fertilizer. We use 10-10-10. Ideally if you have composted horse manure, cow manure, any kind of organic matter like mushroom compost, it makes no difference. Use it. Then cover it up with two inches of mulch or something to preserve moisture. Pine straw, whatever. If you want to use a liquid fertilizer, it’s ok. Usually a tablespoon to a gallon of water. Dump it on and saturate the root zone.

 

At the same time a slow-release fertilizer lasts about four months and that’s all you need.  Typically starting in late march, Hydrangea macrophylla will show new foliage. Fertilize it then, and you covered April, May, June, and July. You don’t want to fertilize after July because all you do is force soft, succulent growth, which is probably going to get injured by early season cold.

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Or use 10-10-10. As I say, 10-10-10 is the cheapest thing you can buy. Inexpensive is not bad when you’re fertilizing your plants. In some places you pay more for the same bag of fertilizer. Plants don’t read the fertilizer bag. The key is get it down. You may only need 4 oz. per plant, depending on the size of the plant. Spread it evenly around the crown of the plant. Don’t put it on wet foliage. Don’t put it on a wet stem. Water it in because it has to dissolve, and let it rip. We fertilize late February, early March and might do it again in late May, depending on whether it looks like it might need a little kick starter.

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• Hydrangeas, particularly macrophylla, need iron. A tablespoon to a gallon of water. This will green them up in a heart-beat. In high pH soils, meaning you’ve got a lot of lime (pH of 6.5, 7, 7.5), they tend to go chlorotic, meaning they’re iron-deficient. You’ll see green veins, yellow in the other areas, and the way to clean it up is to give them liquid iron.

• If you have a freeze coming, cover them. My wife, Bonnie, and I put T-posts, which are mostly used for fences, around the plants and we use string to create a structure. We put cloth over the whole thing and use clothespins to hold the cloth on to the string. You can use frost protection blankets, which provides ~6 degrees of protection. The nursery industry uses it and it’s pretty good. If you have a few plants, put a sheet over them. Do not put plastic on them because cold is transmitted right through the plastic. So put cloth over them, and that will probably save them. You have to be cognizant of when the freeze is coming. In cold weather, 28 degrees F will kill the leaves and youngest tissue while 25 degrees pretty much flattens them outright. Plants like H. macrophylla are the most sensitive followed by H. serrata, followed by H. paniculata. The most frost-tolerant hydrangeas are H. quercifolia and H. arborescens, the foliage surviving 20 degrees F.

 

I always tell my students you’ve got to observe. You build this pyramid of knowledge about any plant, in this case, hydrangeas. Over time you have a pretty good synthesis of what makes a plant tick or not tick or what it needs to survive and thrive and what it’s genetically supposed to do. We’ve tried to breed cold hardiness into macrophylla. We can’t do any better than zone 5, which can reach minus 10 to minus 20 degrees. Once you go to minus 10, you’re going to kill them. We’ve used H. serrata, which is much more cold-hardy. I’ve seen H. serrata flower in Burlington, Vermont when the temperature reached -18 degrees F.  We’re using that to breed a cold-hardy plant. We’ve had a lot of hybrids, but they haven’t been tested yet in zone 4 and 5.  Someone is going to get lucky, just like we got lucky and found Endless Summer®. Someone will find a H. macrophylla blooming in Fargo, ND, Caribou, ME, or International Falls, MN, and that will be the next genetic resource for advancing cold hardiness.

 

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