Reddening isn't from sunburn. But very bright light, short of a burn.

That said, the plants look "hungry" -- as in they need some nitrogen. The evidence is that only the newest growth is a good green, the rest is pale.

Were there flowers on the plants during the spring?
I don't observe any cut stems, so I am assuming not.
It appears that your plants are preparing for dormancy, since they are not producing chlorophyll. The absence of chlorophyll is usually a result of bright light and cool temperatures. Is that possible during your summers?
When a plant stops producing chlorophyll, the everpresent pigments, anthocyanin (red, purple, burgundy, blue) and anthoxanthins (white, cream, yellow) begin to appear. This signals the end of the growth cycle and beginning of a dormant period. Other biological changes occur simultaneously, that blocks the passage of nutrients between the leaf and petiole and the leaf will eventually seperate and fall off.
I wonder if your plants have yet become acclimated to a growth cycle that is 180° from their normal habitat?
You might post on one of the AU Forums, to determine if anyone else has experienced this phenomonom.
Rb

Hi Jean and Razor :)

These plants were propagated from cuttings from my in laws plants. And what Jean said might be right. the summer sun in Melbourne is pretty bright and since the hyadrangeas are in pots on the terrace, where there is no shade, it might be the case of having too much of bright sunlight. I do have some slow release fertilizer which is high in nitrogen and very low on phosphorous. Should I get soluble fertilizers? And also I noticed from reading, most people agree that hydrangeas are suited to fertilizers with high phosphorous .... so I am a little confused.

The plants did have flowers Razor. I deadheaded them about 3 days ago when they were still on the terrace. The flowers look tired and were going brownish.

I did a search on the AU forums but no mentioned anything before. If it helps, it is summer now in Melbourne but the weather has been strange where it can be 38 degrees one day and 19 the next.

Hydrangeas require some sunlight (morning or late afternoon, often referred to as "soft" sunlight), to bloom well. The macrophyllas (yours) also require an even applicaton of moisture during their active growing season. It is generally agreed, that the requirement is about 25mm (1 inch) per week. Those grown in containers will require more, due to the small soil mass and more rapid evaporation.
With higher ambient temperatures, more evaporation will occur through the foliage (to keep it cool) and moisture requirements will escalate.
Fertilization and frequency varies with location. In your area, addition of a slow release, balanced fertilizer can be applied twice each year, early spring and late summer.
Rooted cuttings can benefit from a formula with extra phosphorus. I use a water soluble, 15-30-15, type during the first year of growth and after that, only an annual application of compost and/or leaf mold on the root zone.
Don't know about prices of chemical fertilizer in AU, but the worldwide demand, especially in China & Africa, with limited supply sources, has driven up the price in the US to a point that it is almost unaffordable. Thus, my attempts at organic gardening, both ornamental and vegetable!
Rb

Heya RB!

Thanks again for replying. I dont know if I should mention this, but the hydrangeas you see are still small. They come up to my knee and I am 6'1 in height. Maybe it has something to do with being young. And the leaves are not very big either. I have seen Endless Summer at the same height with leaves three times the size of mine!

Speaking of fertilizers, I have been checking them out and none of the fertilizers that we have here are high in phosphorous. I wonder if it has got to do with the Australian soil. All the fertilizers I have seen are high in nitrogen with phosphorous and pottasium being on almost the same level. Having said that, I did buy some slow release fertilizers today. I'll give you this site and you can check out the fertilizers they have. Just for fun of course :)

www.yates.com.au

athy.

I see what you mean!
Apparently the AU soil is rich in P & K, because there is a note with the ad for Super Phosphate that cautions against using it on native plants.
The formulation for Super Phosphate (0-9-0) is also much lower than here (0-21-0). We are able to purchase granulated balanced fertilizer, with formulations such as; 10-10-10, 12-12-12, 14-14-14, 20-20-20, that many gardeners use for fertilizing everything.
Rb

It was said: " Apparently the AU soil is rich in P & K, because there is a note with the ad for Super Phosphate that cautions against using it on native plants."

Not so.

The caution is because Australian soil is deficient in phosphorus (P). Supplying P can damage, or kill, those native plants.

Hmmmm so should I use the Super Phospate on the hydrangeas along with the normal fertilizer I use since both are water soluble? I am such a novice arent I? And plus the plants we have are not natives ....

Not all AU native plants are able to survive with low levels of Phosphorus(P) and Potassium (K), only those found in arid, sandy areas have adapted. Those growing in woodland and tropical rainforests, need a normal amount of these elements.
With the agriculture revolution and the growing of vast hectares of wheat, the addition of Phosphorus was very necessary to produce viable crops.
The same is true for most introduced plant species, including ornamentals.

Hydrangeas require Phosphorus (P) to produce maximum blooms.
Excessive use of Nitrogen (N) will produce lush, dark green foilage, but at the expense of flowers. Container grown plants must be monitored closely, as watering them will leach all of the necessary nutrients from the soil.
Rb

I have potted Hydrangeas that I transplanted them about 60 days ago. But they do not show any sign for flowering. My hydrangeas leaves turning reddish brown. The substrate of my pots is acidic sphagnum peat moss because I want to have blue flowers. My Hydrangeas are placed in a growth chamber by 16/18 n/d temperature and approximately 18000 Lux light intensity. I do not know why they do not have any flower. I think this problem is due to phosphorus deficiency. I send some photos . Please guide me.

Where are you located and what type of hydrangeas are we talking about? Low phosphorus levels can make the leaves change their color some; no fertilizers?. Why do you suspect this problem? Yes, post pictures but start your own post instead of using this one.