Hi @ChrisNoor,
Something I'll note is that due to the close proximity to the house, you are likely experiencing some issues with reflective heat. Reflective heat is caused when hard surfaces like walls, fences, paving or concrete absorb sunlight and then radiate that heat back into the surrounding area. Even if a plant is not in full sun all day, this reflected heat can significantly raise the air and soil temperature around it, especially on hot days. This can stress plants by overheating their leaves and roots, leading to burning or wilting. I note this because there is a good chance it is playing a role in why your hydrangeas are struggling.
A simple way to help reduce the effects of this is to ensure there is a good layer of mulch covering the soil in your garden beds. This helps to keep root temperatures down and slows moisture loss, which will go a long way during those hot summer days. It will also break down over time, drip feeding nutrients into the soil for later use by your plants. If you don't already have this in place, it could certainly help your hydrangeas if you wanted to give them another year to see how they go.
If you are still looking to replace your hydrangeas, you might like to have a look through this article about Full Sun Plants That Love The Heat as there are quite a few options that you could consider for your space.
Having had a look through the list, the following are the ones I would say would do well and fit that 600mm height requirement.
• Banksia ‘Birthday Candles’ is probably one of the best fits. It stays naturally compact, copes well with heat, and doesn’t mind reflected sun once it is established.
• Dwarf lomandra varieties are very reliable in side passages like this. They handle heat, dry spells and tough conditions well.
• Angel wings (Senecio candicans) work well too, especially because the silver foliage helps with heat. Good drainage is important, but it suits morning sun nicely.
• Society garlic (Tulbaghia violacea) is lower than your target height, but it works well mixed in to soften things, and it copes with sun and heat without burning.
• Coastal rosemary (Westringia) can also work, provided it is kept trimmed. Left to its own devices, it will get taller than six hundred millimetres, but it clips well and handles heat and reflected warmth better than many softer shrubs.
All of these will be far more forgiving than hydrangeas in that position, particularly once the soil is improved and mulched properly.
Let me know what you think, and if you have further questions, please don't hesitate to ask.
Jacob
