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We've used aluminum sulfate to keep azalea blooms blue in the past. This season some new guys apparently missed a fair number of azaleas in a large grouping and we have pink blooms on enough plants to bother the homeowner. Would applying aluminum sulfate now "correct" the color for the rest of the season? If not, is there anything else, short of injection dyes or that kind of thing, that could make them blue quickly?

Edit: The soil is very sandy with little to no clay. Soil tests in neighborhood show 2-3% organic matter content, but these plants get an addition of compost annually so it is likely higher here.

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    Perhaps a generous discount would turn the homeowners color blind quicker than you could turn the flowers blue?
    – Stephie ♦
    Jul 11, 2016 at 12:43
  • @Stephie - sorry for the ambiguity. We apply a layer of a composted mulch called "Sweet Peet" that is very popular around here.
    – That Idiot
    Jul 11, 2016 at 14:18
  • @Stephie - unfortunately (well fortunately for our business) these aren't the kind of clients that are motivated by landscaping discounts. They are more the "We want them blue whatever the cost," kind of clients. And we've been keeping them blue for years.
    – That Idiot
    Jul 11, 2016 at 14:21
  • The ONLY plant I've seen that could be moved from pink to blue is Hydrangea. Never azalea! Go find some annual in color with blue right now. No way can you make red azalea blue. I've never seen a blue azalea. Defined lawn edges, properly pruned shrubs and a bit of annual color will impress any rich person. New mulch...another story but they do like the gag me red fine bark. Best thing are a few large pots with smaller pots (all in the same color or form) filled with perennials and annuals and grouped. Ornamental grasses are a huge hit.
    – stormy
    Jul 12, 2016 at 4:45

2 Answers 2

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Anthocyanins are a common plant pigment. They change from red to blue and purple depending on pH (in fact, anthocyanins were used as pH indicators). The more acidic, the more the color shifts toward purple. I'll pose that the pH in the azalea blooms was made more acidic by the aluminum sulfate (i.e., soil acidity goes farther than just nutrient mineral availability). However, I think the cause-and-effect relationship is that the flowers lost their true color because of an acid deficiency. In other words you can make a purple flower fade to pink (inadequate soil acidity), but you cannot make a pink flower purple (its in the genes).

It might be possible to test my proposition by putting flower stems in solutions of different acidities (analogous to dyeing flowers). I've never tried it, but a pH test kit and dissolving some of your aluminum sulfate in water, might demonstrate this. Another thought is that foliar sprays might work as well. A product called Green Cure is sold for controlling downy mildew. It is potassium bicarbonate, a base (raises pH) that would shift anthocyanin color toward red/pink, if is is adsorbed through the flower stomata. Now, if we just had some blooming azaleas, the fun could begin ;-).

... just an idea.

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  • We're more concerned with making sure subsequent flowers this summer are blue. We can prune the current pink ones out to lessen the impact. These shrubs have always bloomed blue in the past, and most of the other shrubs have blue flowers still. It's just a few annoying ones that have turned pink. But even though the cut stem test won't help us, I am inclined to try it because it sounds very cool.
    – That Idiot
    Jul 11, 2016 at 18:47
  • @ThatIdiot: Applying aluminum sulfate to the soil may be the only thing that will affect this year's blooms (color may be set in the bud, though, which would have happened last fall). But try dissolving some into water in a spray bottle (say, a tablespoon in a quart - spray a pink flower and check in a few days.
    – user13580
    Jul 11, 2016 at 20:47
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You need to check the soil pH around the plants with a meter. If the soil is no longer acidic around only the plants that are displaying pink blooms, then you should add a soil acidifier again though it may take some weeks to work. This can include sulfur dust, ferrous sulfate, and aluminum sulfate. If you've been adding the latter yearly it's unlikely to be an aluminum deficiency. The pH meter will tell you.

https://www.rhs.org.uk/advice/profile?PID=82

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  • So the soil acidifier could work within weeks - rather than months or seasons?
    – That Idiot
    Jul 11, 2016 at 20:21
  • That's what the link says.
    – Graham Chiu
    Jul 11, 2016 at 20:22

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