Q.Can anything be sprayed on large rhododendron plants to kill them without having to cut them?
We are by the sea with 6 acres of field, with invasive rhododendron growth. One bush is about 20 yards long by 5 yards wide. Being able to spray with a long lance and not need to have to cut would save a considerable amount of time.
Yes, you can use a "systemic herbicide" for control of "broadleaf woody plants". This type of material, mixed in the correct proportions with water and a "surfactant" will enter the leaf surface and be translocated throughout the foliage, stems and roots for a complete kill.
With a backpack sprayer you can pump it up to pressurize and with an adjustable nozzle, spray out about 20 feet or so.
In the US for this purpose, I have used Garlon 4 Ultra from Dow Agrosciences, along with MSO (modified seed oil) as a penetrating surfactant. I'm not sure what is available in Ireland, but here is the contact info for Dow in your country. I don't see a "Turf and Ornamental" section, but you can ask.
There are other broadleaf herbicides, and RoundUp with glyphosate, which is primarily a grass killer, will do it also. Keep in mind that your target is not herbaceous weeds or grass, but woody plants. Your product choice and mixing rate needs to be appropriate for the the target plants.
I searched "broadleaf herbicide ireland" and got several companies and products, most are for weed control in grass. But you can contact a local company and tell them what you want to do.
It is an unusual request for us; most gardeners value flowering rhododendrons and want to cultivate them, not kill them. But the same principles and materials apply to control of other noxious woody plants, poison oak, wild blackberries, etc.