Couple of points for clarification - do you mean Pelargonium geraniums, or the perennial cranesbill Geranium? If you mean pelargonium type, are they in pots, and if so, how long have they been in the same pots?

Thank you for your response

They are Pelargonium geraniums, and yes they are in pots.
and have been for several years, although I always dig in some fresh soil in the Spring, albeit a small amount.

It's likely they now don't have enough root room, as they've become larger and more mature, so that either means bigger pots for each, or taking cuttings and raising replacement plants.

You could try a balanced feed such as Growmore, which has as much nitrogen (for leaf growth) as it does potash (for flowers), but I'm not sure it'll make much difference. You could just use a nitrogenous feed, but you might find you lose the flowers after a while, and that's not much good either...

The geraniums are double flowers and my favourite shade of coral which are not easy to find, that is why I have hung on to them. I have and do take cuttings, but nevertheless, the foliage of the new plants never turns out to be as prolific as I would like.

Thank you for your input, it is appreciated.

Can I ask what you feed them with, and how frequently, and do you keep them as functioning plants during the winter months, that is, take them inside or wherever and continue to feed and water and get flowers?

Just a general purpose feed, nitrogen/phosphate/potash mix perhaps once every two or three weeks. I bring them into a greenhouse during the winter months but only give enough water to keep them alive. I don't start the feed until February after I have pruned a little.

do you know the NPK of the feed you use? Flowering plants like geraniums are usually given feeds high in potash to encourage flowering, but if you're keeping them in pots too small for their roots, you may need to alternate between a high nitrogen feed and a higher potash one. Certainly, putting them in larger pots should solve the problem if they're quite rootbound.

Thank you. I will check the Nitrogen content, since it is possible the plant is deficient in this. The addition of more should do the trick.
Do you know of a good product rich in nitrogen please?

I will also check the root system and re-pot if necessary.

Plain old lawn food is high in nitrogen, provided its just feed and doesn't contain weed/mosskiller. Otherwise, something like Growmore, which has a balanced NPK, usually 6-6-6 or 7-7-7, depending on the brand.

I have some Miracle Grow lawn food, which I bought by mistake and have been told that I can use this for feeding plants, but only to use half the dose,

Would it be easy for me to recognise the chemical listed on the product, for weed/mosskiller please? I am not very knowledgeable about this kind of thing :-)

I mean, would it say mosskiller or the name of the appropriate chemical, the latter of which I wouldn't recognise?

Thanks

All lawn treatments list in their description under their name what they do - if its just Lawn Feed, it says just that, if its Feed with Mosskiller, or Weed and Feed, or Weed, Feed and Mosskiller, it says it on the front of the box.