@editor, this is now an example of greenwashing in the very best sense.

We often reach for every more advanced bits of tech to run our homes/offices/transport in more environmentally friendly ways, so it's easy to forget that going greener can sometimes be taken literally. Chez nous, we have several orchids in the bathroom because they thrive in the higher humidity and help mitigate the fact our fan extraction is less than perfect. We normally also have plants on most of our windowsills - especially those East and South facing - but especially early spring onwards since those plants are most likely to be seedlings being brought on early; either that or a forest of basil in preparation for the glut later in the season of tomatoes. We do have a spider plant, a Christmas cactus and a few succulents dotted about but generally our growing is more for eating than for decoration, not that it adversely affects the humidity control.

Having consulted with the plant wrangler in chief, it would appear that most of the plants we have love the sunshine, hence why they are in the conservatory.  The eaves on the house are large (for snowfall purposes I think) so other rooms are much shadier.  It may be a ploy to get me down the garden centre to acquire more for her herd of course!!  But who doesn't like flowers.

Without knowing it, @andrewj, you've laid down the gauntlet. As a result, I've had a bit more of a look into this and enlisted the advice of the RHS.

• The Boston fern I mentioned before is apparently very good for shady spots.
• The cast-iron plant (the famous Aspidistra) is pretty bullet proof and hard to kill, so is fine in dark rooms.
• Begonias normally grow on dimly lit forest floors so ar already naturally suited to rooms that aren't all that sunny.
• If you like cacti, Rhipsalis hails from shady rainforests, so tolerates a fair degree of shade and, as a bonus, most varieties don't have spines.
• The Wallis peace lily is best grown in partial shade.

As you say, your beloved's strategy for further adoption is clear; that plant centre trip is looking more likely by the minute and I'm not going to apologise.

@majordennisbloodnok I’m glad I posted this.  There seems to be a general agreement here that a few more plants will go down well.  Thanks for the suggestions - aspidistra went down well!

That’s my pleasure, @andrewj. The only challenge now is finding the room for the new additions. Coffee tables? Free standing shelves? Ceiling mounted hanging baskets?

I’d probably better not mention automated watering systems 😉

Posted by: @majordennisbloodnok

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I’d probably better not mention automated watering systems

Can you link them to Home Assistant so they can be intelligent?

Posted by: @jamespa

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Posted by: @majordennisbloodnok

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I’d probably better not mention automated watering systems

Can you link them to Home Assistant so they can be intelligent?

I said I wouldn’t mention them, @jamespa.

So I won’t mention that yes you can, along with moisture sensors to decide when those systems need to kick in.

There. That’s that not mentioned.

@majordennisbloodnok Leaf it at that perhaps?

Yes, ours dropped by about 25% RH. I figured it was a combination of upgraded ventilation  (DMEV in every “wet” room and trickle vent on every window) and 24/7 heating. Not had a single instance of window or door condensation al winter, first time ever. 
We get along ok with 40% RH, better than too wet, it’s only for a month or so longer and the heating can go off and the windows can open. Quite excited today, our PV output was higher than our heat pump input, first time this year.

My 2 bed mid-terrace definitely dropped consistently after the heat pump was installed. Previously, because I still have single glazing, I used a small window vacuum to remove the condensation in the morning. I haven't used it since. 

Humidity has been as low as 45% when near zero but currently back in the 50s now it's 12C outside I assume.

@andrewj, I was wondering if you'd had any update to your humidity balancing endeavours. Have you been on a horticultural spending spree? Does the house feel better now?

@majordennisbloodnok I haven't no.  However, I have changed the location of the thermostat to the living room as in this warmer weather leaving it in the cloakroom, with it's higher heat loss, was just overheating the rest of the house.  In its current location, heating isn't on as much with the result that humidity is around 45% - 50% which is a lot better.  We'll almost certainly get some foliage at some point though!