Hydrangea Growing for Fun and Flowers!

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The purpose of this cultural sheet is to act as a beginning point for those of you who would like to try their hands at the relatively unknown art of hydrangea culture (forcing) which yields plants that will bloom during their first year of growth in a table or window sized pot. If your aim is purely for landscape purposes, than I suggest following these instructions until first frost. In zones 7 - 10, mulch over your plants in a protected area of the garden and transplant in the spring after growth resumes. For zones 5-6 I suggest over wintering in a shaded unheated structure, room, or basement. If your North of zone 5 you had better rethink Hydrangea macrophylla in your landscape, they might survive but will seldom flower. They will do fine as container plants (12-15" pot) as long as they are brought under shelter (garage, unheated room, wood shed, etc.) every fall. I suggest either Hydrangea aborescens "Annabelle" or any one of the fine Hydrangea paniculatas for the landscape in zones 3-5. In all cases you must be sure to never let your hydrangeas dry out. They will lose water even when apparently frozen through the process of "sublimation", where water goes from ice to vapor without turning into a liquid through the action of wind and dry atmospheric conditions. Be especially careful in late winter, early spring. This winter kill should no longer be an issue for plants planted out next spring, as they should have developed an adequate root system by the following winter. If you plan on growing your plants inside for the duration, and your not interested in forcing hydrangeas as a hobby (in my case, an addiction) than you can pretty much ignore most of my ramblings providing you have some green in your thumb. In any case, you will be seeing all phases of growth from bud formation to first color, which can be just as fascinating as flowering.

Pot Selection

The first thing you will need to do is transplant up to a 3.5"-4.5" pot. Round or square is fine in either plastic, fiber, or clay. Clay promotes the best root growth because of its porous nature which allows some air circulation. It has the distinct disadvantage of drying out rather quickly requiring frequent watering and leaving little room for error. Fiber is a little better than plastic as far as aeration, and requires less watering than clay. Adequate drainage holes are essential with all types. It is better to step up wen transplanting, growing out in a smaller pot until the roots have filled it, rather than transplanting into a large pot right from the start. This will promote better root growth with less chance of stagnant or water logged soil, and the resulting higher incidence of root diseases.


Just about any soiless mix is a good beginning. I've been using a coir/peat/bark/sand/perlite mixture which I cook up according to taste and availability. My favorite recipe is 50% coir based soiless mix, 20% peat, 20% native Nantucket sand, and 10% coarse perlite, with a little wetting agent or water crystals thrown in for good measure. The perlite, which is expanded lava that most people think looks like Styrofoam, helps keep the media from packing. Lava rock, coarse vermiculite,, or pumice will also do the trick. It also promotes good drainage while adding a few basic minerals and aiding in moisture retention. I use sand to help with drainage and act as a source of minerals, trace element, and metals such as aluminum which are necessary for blue flowers. I would use 20% native soil if didn't have sand, adjusting with more perlite if it has a lot of clay, or tends to pack and form clumps. Of course most people will do just fine with a straight peat or coir mix, I just need to add my 2 cents. If you would like to add fertilizer or compost at this time, by all means do so. One word of caution when using native soil. At this young age, hydrangeas and other shrubs can be susceptible to various root diseases such as pythium which can be found in soil that has not been treated. Watch for brown root tips and treat with a fungicide such as Clearys or Banrot. You should see noticeable growth and renewed vigor within 2 weeks of transplant if all is well below the surface. Don't be afraid to check the roots, they should have new, white tips, not dead or rotting brown ones which is usually caused by fungus or over watering.


Hydrangea - [Hydra]=water; [angea]=vessel or container What's in a name? With hydrangeas the right amount of water at the right time can mean the difference between success and disappointment. You must never let them dry out... this may result in permanent damage to foliage and flowers, or worse. Many factors such as temp, wind, sun, and phase of growth will determine how much and how often to water. Ideally one will water just after the media dries out but before the plant shows any sign of stress. Southern growers may experience wilted leaves and flowers in the heat of the day (90 +) but the plant will recover during the cooler evening hours with no apparent ill affects as long as there is water in the soil. Let the soil dry out and the same wilted leaves will burn and not recover. If this happens often or for extended periods the plant may die. This is not as big an issue with an established landscape plant although they will astound you with the size an number of flowers if you irrigate on a regular basis. Hydrangeas are particularly susceptible to water deficiencies during their bloom period and will often have to be watered everyday, or more if left in a small pot. As mentioned above too much water will inhibit growth and lead to fungal diseases. Too much rain water can cause problems with containerized plants when grown outside if there isn't proper drainage. It will also leach out fertilizer and other nutrients. Watering in the morning or early afternoon will avoid wet foliage during the night and help to minimize fungus and mildew problems.


The optimum scenario for hydrangeas is to keep them growing as fast as you can without making them overly lush in the process. Yea, I know, another paradox. They really like their fertilizer and can be fertilized with every watering, once a week, by adding fertilizer, manure, compost, or Grannies secret elixir to the potting mix, or with time release granular additives (Osmocote). I use 1 tablespoon per gal for weekly watering or 1 teaspoon per gal for "continuous feed" (fertilizing every time you water) of a water soluble 20:10:10 (20:5:10 for red or pink flowers) granular fertilizer with elevated micro nutrients (peat formulation or "Miracle Grow"). It's important to "leach" out your plants every couple of weeks to avoid salt buildup, especially when adding "aluminum sulfate" (see "ph and Color" below) for blue flowers. Fill your pot to overflowing with water until excess begins to drain out the bottom. Chelated iron can be added every 4-6 weeks as directed to avoid iron deficiencies which may lead to "chlorosis", the yellowing of leaves due to the absence of chlorophyll which requires iron to form. The following is primarily for commercial growers and can be ignored without loss of sleep. For those of you with the luxury of a dosatron (fertilizer injector), 100-200 ppm Nitrogen continuous, 300-400 ppm N intermittent from the above sources or their equivalent. If you have an EC meter (measures dissolved salts) try to maintain media between .1 and 2.0. I haven't any personal experience with soil amendments or additives but if you would like to use them try to maintain Nitrogen at about 75 ppm in dry soil.

ph and Flower Color

One of the most charming traits that hydrangeas have is their almost singular ability to change color depending upon soil type and ph. What's even better is the fact that you can control this phenomena, coaxing colors that aren't usually seen in nature. I've seen electric blues and psychedelic purples that are simply stunning along with some of the purest, clearest pinks, reds, and blues, sometimes in combination within the same bloom or plant. There are 4 things that I know of that can account for color changes in hydrangeas. The first and most important is the variety of hydrangea being cultivated. There are certain cultivars which will always be red because of their genetic makeup, and others that will almost always be blue if they can get some aluminum. This trait isn't as intractable with Hydrangea macrophylla (all 6 varieties received are macrophyllas) as in some of the species like Hydrangea serrata. Some varieties just want to be blue, and some would rather be pink. Blaumeise will readily go either way, Mathilda can have stunning purples and great mauvish reds but is usually grown as a blue. I've never grown either Enziadom or Blue Danube as a pink, might be interesting... hmm. The second is the amount of aluminum in the soil which makes the red pigments (delphinidin 3-monoglucoside) in a hydrangea flower turn blue. Aluminum sulfate will provide this aluminum if blues are desired as long as the ph and other minerals are present in the right proportion. The problem with aluminum sulfate (alum) is that it's a salt which can be lethal to any plant at high doses. I once accidentally mixed at batch at 10 times the recommended rate and put them on Enziadom, supposedly the bluest hydrangea available. Most of the plants died, but you should have seen the flowers on the ones that survived! If I didn't have Nantucket sand to grow in and was determined to have blue blues, I would add 2 oz (4 tablespoons) of aluminum sulfate to 1 gallon of water, watering the plants (4" pot) with 1/2 cup of that solution 2 times during this season, waiting at least 2 weeks after transplant before applying, and 2 weeks thereafter. Applying alum after the flowers have started to color won't change a thing except possibly causing "abscission" (dropping) of the lower leaves. The 3rd thing is the soil ph, generally ph = 5.0 to 5.8 will get you pinks or reds, 4.5 to 5.2 will be best for blues. Purples come somewhere in between depending on the variety. If you grow in a commercial potting mixture, adding the aluminum sulfate should take care of the ph, since it lowers it by being acidic. Never let your ph fall above or below these limits as death may result. The 4th factor is the concentration of other minerals in the soil such as phosphorous. Certain minerals such as phosphorous, will prevent the uptake of aluminum. Try to use a fertilizer that's low on phosphorous for blues (the P in NPK), such as (20:5:20) Other minerals and metals assist the uptake or take the place of aluminum with varying results. That's why the old wives tales of adding Epsom salts (magnesium) or burying a nail (iron) sometimes work. You really don't have to bother with any of this if your not particular. Most commercial mixes end up on the pink side of things. By adding soil or sand you may come up with just the color you want. The stately whites, Souer Therese and Regula, are indifferent to all this, and perhaps you will be also.

Light & Temp

Optimal temps are between 65 and 85 Fahrenheit. Avoid frost. Freezing (less than 28) will likely end your growth for this year. They like sun to develop flowers but also need some shade to avoid being fried. I would use 30-40 % shade at this time of year in most zones. The closer you can get to these ideals, the better for rapid growth and healthy plants. A fairly sunny window sill open to the air occasionally will do just fine.

Floral Initiation or Vernalization

Many plants require seasonal environmental conditions similar to those found in their native climate to initiate floral development. Hydrangeas form next years flowers during the fall of the previous season in their natural habitat, but they are pretty easy to trick in this respect. In fact they can be brought into flower at almost any time of year. Since these plants are being grown during the fall, working with nature rather than against it, you shouldn't have to worry about any of this in the mid latitude zones (6-8), as long as they are being grown entirely inside throughout the winter or out of doors until November. For points South of 8 they may need to spend nights out of doors or in a relatively unheated and unlighted room over the next few weeks to help flower formation along. Zones 9 & 10 may require refrigeration, but will eventually have flowers without it, although not of the best quality. North of 6 will have to protect from frost/freezing. Here's the deal. Once temperatures have dropped below 65 for 5-7 nights and sunlit day lengths get shorter (8-12 hr), hydrangeas will naturally begin floral initiation, a fascinating process which isn't over until they resume growth in the spring, or in the case of no dormancy (indoor culture), 2-3 months later. This can only happen if the plants have developed to a point where it can physiologically handle bud formation (5-7 weeks from root formation). Your plants already have about 4 weeks of "vegetative" growth and should be ready to begin floral initiation in 3-4 weeks. Try to keep them somewhat warm until then in the more Northerly zones. If you live in the southern zones you may have to put them under air conditioning for a few weeks. A fully developed fall bud will have a "rugose" (wrinkled) appearance rather than a smooth exterior. Of course you can always slice a bud in 2 lengthwise and look at it under a microscope where the fertile flower primordia should be visible.

First Frost

Frost should be avoided until all the flowers have made up, extending the growing season artificially until around the first week of November in the South (7-9) or the middle of October up North. I've lost nicely budded plants when the first frost becomes the first freeze. On the other hand, if your plants have been hardened off by sun, wind, and cool nights(30's), and they have made up bud, they should be able to take it. In fact they will be better off for it if they survive, and will have to put up with freezing temps anyway if you will be over wintering outdoors. A cold frame should work in the north. I sometimes use a sheet, tarp, or drop cloth on iffy nights, being careful not to lay them directly on the plants which might result in broken buds. For those of you on the window sill, make sure you close the window when frost is called for as your plants won't be quite as tough as those raised out-of-doors. This is the point where most people can bring their plant indoors if they haven't already done so, pretending they're just some other house plants, patiently waiting for opening buds to appear which will probably take another 2-3 months. If your going to wait until next spring to plant them in the landscape without flowering, than now is the time to get them under cover. If your really serious about learning how to "force" them, that is getting them to bloom at a specific time like Easter, than read on.


One advantage of a frost is "natural defoliation". "Natural defoliation" reminds me of the year that I left some young plants out during a frost to have them defoliated before bringing them into a unheated greenhouse for the early winter. I guess the deer like "naturally defoliated" vegetation because they trashed every last one of the plants, stripping the frozen leaves right off the plants. I stormed off that morning thinking the crop was ruined and didn't have the heart to look closer for about a week. Funny thing was, when I took a good look, 95% of the buds were still intact, saving me my crop and from having to strip them myself. Every time I picture them daintily stripping the leaves while leaving the buds behind I get a pretty good chuckle.

Dormancy Not necessary for indoor culture, only for timed production - see forcing link below. The following topics will be added to or are covered by the links below.


Growing out


Forcing link:


Propagation Link: