Has it grown well until now?
Does it grow in full sun?
Has this plant been fertilized?

1. It has the dimensions of a full grown shrub. It did flower well once after I treated the soil with aluminium sulphate because of my guess that alkalinity was the problem. Whether this flowering was because of the treatment or just coincidental I don't know. That the soil is alkaline seems to be certain because of the pink flowers on hydrangeas growing nearby..

2. It is in full sun.

3. The only furiliser used is the aluminium sulphate, as above. It certainly has had no phosphorus.

What I would do now would be to spend about $20 and get a soil test kit and test your pH level. Aluminium sulphate is O.K. for lowering the soil pH, but you to know what it is at the present time before any further treatments of any kind.

Grevillea Superb will also take a good "haircut" once a year and will help thicken up the plant. This is something I did on a regular basis with my Superb which helps keep it healthy.

Thanks for your interest.

However I am only concerned to discover whether alkalinity is the factor. If so, I will replace the shrub with something more suitable rather than spend the rest of my days adjusting the ph.

I will certainly give it a good prune to see if this has the required effect.

Again, thanks.

I guess it depends how attached to the Superb you are.

I doubt very much that pH is the cause of your problem. If as you say, the plant is showing normal growth then pH would seem to be fine. If you have a pH (iron deficiency) problem then the typical evidence would be stunted growth and pale yellowing on the leaves especially the new growth.

How old is the plant? It could be that there is an excess of Nitrogen in the soil. If the plant is only a few years old then time will result in flowers. The plant will eventually exhaust the soil of those growth nutrients like Nitrogen, as long as you do not feed it for a while (until it flowers).

Some Potassium can encourage flowering. Back off on the watering as well.

A very belated follow up!

Thanks for previous responses.

I had more or less given up on this shrub, and just left it to its own devices. Think I have pruned it once or twice in the meantime with no result of any consequence.

In a rush of enthusiasm a few weeks ago, I decided to dose it with potassium, and it is now flowering quite well. So all's well... But perhaps it would have flowered anyway!