Alright Dean,
About 5 years ago I had the same idea. I'd just planted £2,000 worth of hedging and the customer had specified large plants. I couldn't believe the prices and so thought 'this gives me an idea.....'
Next door is a nursery and so I rented a polytunnel and started growing a mixture of mainly hedging plants i.e. aleagneus, escallonia, pittosporom, acuba, laurel, forsythia..... etc etc
My plan was to grow it big so that people could have an instant hedge and I could have lots of money!
However several things that I didn't take into consideration:
The market - what were people really after and was I growing the right things?
The job of potting plants from a smaller pot into a bigger pot gets tedious after the first 500.
The hire of polytunnels and cost of water adds a lot onto your plants after a few years.
There is a lot of maintenance to do i.e. weeding of pots and tidying up.
If doing it in any sort of numbers to make an income you'll have to employ people and this makes serious dents in your cash flow.
But hang on a sec!? When we want a large tree or shrub we go into a garden center and we say 'how much?' - my word they're making some money out of this!
The trouble is unless you have access to a market e.g. you are a landscaper and use a lot of plants, or you can produce enough to go down the wholesale route where are you to sell them? People only pay high prices for plants at a nursery. When they come over to you they expect to pay half the price they would at a nursery.
People love nurserys, they love the coffee shop, the browsing, the pushing round of those little two wheeled trolleys, buying gloves for £10 that would cost £1 at a market, the pleasurable wasting of a Sunday afternoon....!
So sell them to a nursery? Most of the big ones grow their plants themsleves or import from Holland (where they really do cost peanuts). Why sell yours when they need to sell their own? If they do buy them it will be for around 40% of what they sell them for.
And loading and delivering the things takes an age... To get 10 large plants out the field and onto the truck, take them to the customer unload and have a chat - you could have made the same money doing a tree job (NOT includig the growing!).
And the best thing is no matter what you grow they'll come round and say 'have you got any such and such'. Which of course you haven't.
I think I made my money back just (I tell myself that anyway) but the huge amount of time and energy that I put into it never returned!
I sold the remaining off cheap last year for £5-00 a plant (some were 8 foot tall) and have been breathing out a sigh of relief ever since!!
That was my experience anyway but please don't let it put you off!