The water from it won't kill it, but every time I've seen (from the outside) one of these boilers working, it produces clouds of steam - that hot water/steam won't do the plant any good at all.

it may sound like a good idea but leave the pipe as it is. it should be well out of reach of fingers if the pipe gets blocked for some unknown reason what is going to happen..when the boiler lets of steam pipe blocked pressure release valves blows so where is the boiler attic or elsewhere. you working in the area may get badly burnt.
sorry for going on just thinking about your welfare.

Whilst the condensate is possibly harmless to plants ( I am not so sure about that) interference with the flu could cause all sorts of problems for the boiler, as Kenny says, not to mention invalidating any warrenty. I would leave well alone.

combi boilers or any boilers , gas heaters are of a very fine tollerance ie the egshaust . i realy recomend you totaly forget this idea as it coulld be very VERY dangerous .kenny is more than right .

We approached the manufacturer of our dead boiler about a replacement. They sent us the names of plumbers, corgi registered (now something different cr what). We asked several to call. The one who came was a disaster. To cut a long story short they thought they could channel the condensate in to our Septic Tank. Fortunately we read the literature which said that was a definite no no. If you cannot put it in the septic tank I would not let it near any plant of mine. Good news is we found a Scottish company who knew the law and we were able to replace like with like. The Corgi plumber they supplied was first class and will attend to the servicing every year.

Taken from Wikipedia

Exhaust
The condensate expelled from a condensing boiler is acidic, with a (pH between 3 and 4) about the same as an orange. Condensing boilers require a drainpipe for the condensate produced during operation. This consists of a short length of inexpensive polymer pipe with a vapour trap to prevent exhaust gases from being expelled into the building. Though the mildly acidic nature of the condensate poses no health risk to occupants, it may be corrosive to older cast iron plumbing waste pipes and concrete floors. A neutralizer is employed in such cases, typically consisting of a plastic container filled with marble or limestone aggregate or "chips" (alkaline) to raise the pH to acceptable levels. If a gravity drain is not available, then a small condensate pump must be installed to lift it to a proper drain.

The primary and secondary heat exchangers are constructed of materials that will withstand this acidity, typically aluminum or stainless steel. Since the final exhaust from a condensing boiler has a lower temperature than the exhaust from an atmospheric boiler 38°C (100°F) vs. 200°C (400°F) a fan is always required to expel it, with the additional benefit of allowing the use of low-temperature exhaust piping (typically PVC in domestic applications) without insulation or conventional chimney requirements. Indeed, the use of conventional masonry chimney, or metal flue is specifically prohibited due to the corrosive nature of the flue products, with the notable exception of specially rated stainless steel and aluminum in certain models. The preferred/common vent material for most the condensing boilers available in North America is PVC, followed by ABS and CPVC. Polymer venting allows for the added benefit of flexibility of installation location including sidewall venting saving unnecessary penetrations of the roof.

From the above I think there is a flue and a condensate drain pipe. They are different in that the flue can be seen as a side wall vent but the condensate must exit the property via a drain either gravity or pumped.

Thanks for the extensive info, Scotsgran, very interesting. I'm still not clear about boilers myself really, I have a combination boiler, my neighbours have had a 'condensing' boiler fitted, but I think there are now more modern versions of even that - I'm still confused as to what an 'atmospheric' boiler is in comparison to a condensing one.

I am not a technical person but my husband is. He got hold of the data required to decide what was necessary. If you put the name of your boiler in to your browser then follow up with Q's on what each of the jargon words mean it will help. Our CH is oil fired and I cook on a coal cooker, an Esse, which I do not want to part with. Our gravity fed system works wonderfully well.