Hi Kelly,

Generally, the things to look out for when a gardenia starts to fail out of the blue are potential problems you might not be able to see on the surface. I may have a few suggestions, so forgive me in advance for the longwindedness :)

How's the drainage in the general area where the shrubs are planted? Gardenias like very good drainage for sure, without too much fluctuation in humidity and air flow throughout the plant. You can also try to supplement trace minerals, particularly magnesium. Epsom salts are good for this, as the sulfur also acidifies the soil. It's not a quick-fix-silver-bullet type of thing, because it can take weeks or months to see improvement.

Are you for certain the soil is also acidic enough? You mentioned being planted underneath an oak tree, but oak leaves might not be enough. An acidic fertilizer (like for camellias and azaleas/rhododendron) being applied might improve acidity, but it's always best to test your soil pH to be sure.

Also check to see if there might be tiny pests, particularly if there's any webbing on the undersides of leaves. It could be spidermites or some other juice-sucking critter.

The gardenia may also benefit from removing any of those oak leaves around it and replacing with a bark-type mulch. Those leaves can harbor pesky problems and create too much fluctuation in moisture, temperature, humidity, etc.

Even the best cared for gardenia can have some yellowing leaves. It happens to mine sometimes, particularly a month or two before they start to bloom (they bloom here in May). My theory as to why is that the lack of acidity, even when slight, prevents the plants' roots from properly absorbing the phosphates in the soil to create buds and the yellowing leaves are just the plants reaction.

You may also have to give the gardenia a year or two longer to really establish itself. They're known to have a long-term (a year or two sometimes) ugly phase when transplanted from one environment to a completely different one.

Hopefully some of these things will help. Good luck! :)

Hi Jason, thanks for your reply! The location of the gardenia has pretty good drainage, though it is in clay soil. When we bought the house last year, the previous owner had just done a soil test and told us the soil was pretty acidic, though I don't have the test in front of me to verify that. However, I watched the neighbor's hydrangea a few feet away change from pink to blue over the course of a season last year, which tells me that the soil has at least a certain degree of acidity. I have some epsom salts and fertilizer that I'll try once it stops raining, and hopefully that'll stop it from sulking! Otherwise I'll keep an eye on it and maybe it's just going through the ugly phase like you said. When I bought it, all I was thinking was how much I loved gardenias; I had forgotten how finicky they could be!

Thanks!
Kelly

Kelly,
Gardenias are not finicky at all.
You are too cold for ground cover gardenias where you live, zone 6B.
Gardenias in the right zone are one of the most hardiest bushes around.
Try growing them in cute containers under your tree, and put them inside during the winter where you live.
Don't baby them. Water them once a week, or if real hot a couple times a week, and use regular Miracle grow on them about once a month in summer.
That's it.
Hope they made it.