It's likely the half moon containers don't contain enough depth for the roots of even one full grown trailing nasturtium, they're usually pretty shallow, so try with just one seed in each, or two and pull the weakest one out as they start to grow. Also note that nasturtiums do much better in poor soil, so if you're using expensive potting compost, don't, potting compost from last year will do, provided nothing infected was growing in it.

12ft is quite a stump! I would drill lots of holes into it and try and excavate some space behind it to allow a decent planting space. Nasturtiums do not have much root in them but do like moist poor soil to sit in. I would place one seed/ling per hole. I take it you have the trailing variety and not the standard ones that trail through rather than down?

Have you put something to hold them up round the stump? They won't hang on by themselves.

(Why not use clematis instead?)

Personally, I'd chop the 'stump' down to something considerably less than 12 ft, the only things that are going to cover it, otherwise, are a vigorous clematis or rambling/climbing roses.

Thanks, all. Interesting interpretations of question. 12 foot stump was specified by me when having a storm-damaged pine cut down - with intention always of covering with nasturtiums. Containers - 6 in number - are quite large, maybe 14" x 7", and are hung around stump from encircling cables. They give me at least 6 inches of soil depth. Seeds are sown in propagator, then planted out. As suggested, problem may lie in having so many plants and giving them insufficient spacing. 2 or 3 only per container?

Could we see a photo? It's hard to imagine!

One in each...

Hi, Landgirl100.
I'm new to this website and couldn't see the means of including a picture in a comment such as this. I have uploaded one to the site with a tag of "nasturtiums". Hope you can find it. Sorry, but without the floral coverage the set-up looks pretty unattractive, but last year it was quite effective; I know, however, that I can make it superb, with proper planting. Thanks for interest.

Hi, Bamboo!
Just read your very brief but very positive comment - also saw your impressive credentials in your profile.
One plant per container.... Wow! Food for thought. With all respect to your advice I shall plant 2 planters with one plant each and compare results against others with 2 and 3 plants per. I'll know for sure what to do next year. Thank you.

Hi, Bamboo!

Thanks for impressively brief and authoritative comment. Your credentials are most impressive, and I must take them on board. My plan is now to plant two planters with one plant each, and the remainder with either two or three; by comparing results I should know exactly what works best for 2016. Looking forward very much to seeing outcomes later.

I'm trying to visualise your containers and the volume of soil they could hold - I'm assuming they're curved inwards from the top at the sides and front down to a much narrower girth, and you have said only 6 inches deep of soil. That's what I'm basing the one plant per container on, although to be honest, I might chance two in some to see what happens... but certainly not more!

I'm off to try to find the photo you've uploaded, but what I'm really interested in is seeing the containers...

UPDATE: found the pic showing the containers, they're not as small as I assumed from your description, yours are more window box shaped; my half moon planters were significantly narrowed as the went down to a 4 inch bottom end. Two in each (though curiously, one looks to be a different type and a bit smaller than the others...) though I'd still plant a couple with just one to see if they actually get longer and bushier than the ones with two in a pot - they should do.

I'd like to see photos please, later on when they've grown. By the way, have you ever considered Convulvulus sabatius for a change? 150 cm long by 90cm wide when grown, one to a pot - only useful if you like lilac flowers though... but in a mild winter, they often survive and carry on the following year. You could alternate each box with purple surfinia petunias, - fed weekly and kept watered, they can get as long as a metre. Neither of these is as inexpensive as planting nasturtiums though, but they will be in flower much earlier and go on as long, provided you buy plants, not seeds.

'Impressively brief' hmm... yep, I can be, but also the opposite, as you've discovered here...