Shade and fertilizer are two of the main reasons for azaleas not blooming.

Shade prevents bud set for many varieties. Opening up the shade or moving to a sunnier location will improve bud set. Girard Rose prefers partial to bright shade or morning sun only, as the flowers fade quickly in full sun. If shade is too dark it will not have good bud set.

Nitrogen containing fertilizers promote foliage growth at the expense of bud set. I don't fertilize very often. If you must fertilize, only use a fertilizer with an organic nitrogen source such as Holytone and only use it once per year around bloom time and at half the rate on the package. Sometimes less is better.

Azaleas that do best in shade include: calendulaceum, kuisianum, and schlippenbachii.

Here is a link that might be useful: How to care for rhododendrons and azaleas.

I only fert with Hollytone once a year, so I don't think they're getting overfertilized.

Last year they got lace bugs. Do they hurt the blooms?

Deanna

Lace bugs don't normally harm the blooms unless the amount of healthy green leaf area is reduced too much. Lace bug can result from spraying and killing the predators that control lace bug, or placing a plant in too sunny an area. Lace bugs reach their peak in late summer and do their worst in sunny, exposed sites. Spiders are important predators of lace bugs and since they shy away from sunny, hot places, keep your azaleas where there is some shade.

Well, then that doesn't make any sense at all, because the shrubs are in full shade. I don't spray insecticide, so no predatory insects were killed.

Deanna

Deanna,

The azaleas need some sun, morning preferred but a few can take full. I have four, two being from the Girard brand. I don't fertilize at all, I give them half a cup of AlSO4 or NH4SO4 after they have bloomed. You will need to move them or enjoy them as evergreens because they will not bloom in complete shade.

Full shade means few if any flowers. Full shade also means that you most likely didn't have lace bug, or you need more beneficial insects.

Do not use Aluminum Sulfate: If you google "aluminum sulfate rhododendron azalea" you will find some extension agents still recommending aluminum sulfate while most are warning against using it. It is sad that such bad advice is still being given.
Never use aluminum sulfate for making the planting medium more acid. Thousands of azaleas and rhododendrons are killed each year by the addition of aluminum sulfate to planting mediums. Aluminum ions under very acid conditions are very toxic to all of the rhododendron genus.

Aluminum is not considered to be an essential element for plant growth. In fact, for most plants, high levels of available soil aluminum are toxic causing stunting of root growth and eventual death if soil aluminum is high enough. Usually the soil pH has to be less than 4.5 for this to happen. Rhododendrons and azaleas are more vulnerable than many other plants.

Although aluminum sulfate often is recommended to gardeners for increasing the acidity of the soil, it has a toxic salt effect on the roots of plants if it is used in large amounts. Small amounts are not very effective. About 7 lb. of aluminum sulfate are required to accomplish the same effects as one lb. of sulfur.

The one area where aluminum sulfate is recommended is in making blue hydrangeas blue. The chemistry of hydrangeas is such that not only acidity is necessary, but aluminum ions are also necessary to make the flowers blue due to the aluminum binding with the anthocyanin. Hence, blue hydrangeas shouldn't share the same beds with rhododendrons and azaleas. Over application of aluminum sulfate can be toxic even to hydrangea.

I am sure that part of the reason for the bad advice is that aluminum sulfate is very quick in modifying the soil pH. Sulfur is very slow, but is much more effective eventually.

Guy Nearing was one of the first to realize that aluminum sulfate was detrimental to rhododendrons and azaleas. His findings were published in the Journal of the ARS in 1955.

Rhododendrons are Vanishing by Guy Nearing

Today, the effect is thoroughly understood. The most eloquent article on adjusting soil pH for rhododendrons and azaleas was written by Sandra Mason with the University of Illinois, Champaign, slmason@uiuc.edu.

How to Lower Soil pH (and how not)

In her words, "Many acres of land in the world are unusable for crops due to soil acidity and aluminum toxicity."

rhodyman,

That might be the case but I have clay soil and have used 1/2 cup of AlSO4 on my azaleas/rhododendrons for over nine years and I haven't lost a plant yet. I on my last bag of the stuff and the stores now sell NH4SO4 which I switch too when I run out.

You are using a substance that is known to be harmful to azaleas. And you want to use up this substance. I don't understand. Fortunately Aluminum Sulfate is poor at increasing the acidity. If you use something that is good at increasing the acidity while the aluminum ions are still in the soil, you will kill your plants.

I would never use Ammonium Sulfate either. It is a chemical fertilizer and is a very poor source of acidity. Your azaleas don't need much fertilizer. Cottonseed meal is a much better source of nitrogen.

By the way, have you ever tested your acidity. It can't be very acid or the aluminum sulfate would have killed your plants. I wouldn't do any acidification for a while. If you acidify too much with the aluminum ions present you will slowly kill your plants roots.

The best and longest lasting method to acidify the soil is powdered sulfur. But wait a while. If your plants have low acidity problems, you will see a yellowing in the leaves between green veins. Then iron sulfate and sulfur are your best bets at treating the problem.