Okay two questions if this is going to be indoors will there be natural light, even 'weeds' need light and how are you going to keep them watered?

Well, the fastest plants that I know of are radish seeds. They'll sprout in 2-3 days, but it may be hard to keep the soil moist enough without it smelling moldy. Making ground coir--sometimes sold as "coco peat"--and activated charcoal part of the mix may help with that.

Hello,

Thanks a lot for these answers!

So yes it is going to be indoors in a gallery type environment with almost no daylight! Sorry i should have mentioned this.

There will be people on site able to water the sculpture from time to time during the show.

Thanks a lot Tugbrethil for these tips!

Light could be a problem, as Moon_grower says, so you may have to remove mangy seedlings and plant new seeds fairly often through the show.

You'll be frustrated trying to keep live plant for 2 weeks. Consider using 'life-like' silk flowers or 'life-like' wax plants, foliage, etc.

Trying to keep live plants going for two weeks with no natural daylight is not going to work! You could grow the radish seedlings in pots and sink these into the soil and sand but even then they will not look great. Once the sculpture is open to the public will you have access to? It is going to need daily care and attention to keep thing going and I'm not getting the sense you are any sort of a gardener. Sorry if I'm wrong on that point!

Artificial flowers an option?

Natural flower seed heads such as small allium, clematis etc ?
Or if mushrooms would be acceptable would they grow fast enough?
Or could you sink trays of cress, such as you can buy in the supermarket?

You might be able to turn it into a giant Chia pet with chia seeds, which sprout quickly.

Mustard and cress perhaps?

All plants including mustard and cress need natural light to grow. Puddingo take a look at the range of, almost, natural looking plants, such as ivy, in the artificial plant area of your local GC.

Maybe some grow lights can be suspended over the work. There is one plant in the hemp family that seems to thrive inside ;)

True Moongrower - must confess I was scraping the bottom of the barrel there...seed heads is a good idea but sourcing enough might not be easy. Wondering about the feasibility of getting lengths of plastic ivy and cutting it into segments to stick in the soil as though it were growing?

That is why Puddingo needs to look at artificial plants in GCs