Devon,
United Kingdom 
I have received 5 plug plants of Verbena Bonariensis and they are about 3" tall and very stocky. Why does the grower say they should be potted on before planting in the garden? They are much bigger than the seedlings I have left to grow in situ in the past. Does potting on help plants to grow? I never understand why we are told to pot on and on until plants are a certain size. What happens if one plants plugs straight into a large pot? So two questions really.
27 Apr, 2012
Answers
Regarding your second question if you over pot a small plant it is likely to end up sitting in a lot of cold, damp compost. Potting on allows you to encourage the plants root system to grow a little more not suddenly feel lost.
Copy and paste the link below into your web browser
http://bit.ly/Iwj5kQ
Excellent video by RHS explaining potting on
27 Apr, 2012
I lifted my Verbena b. and put it in the greenhouse over the winter.....it really doesn't like cold weather. I will put it out in May.
27 Apr, 2012
Most of mine have come through the winter outside - and I have at least 50 seedlings popping up in the troughs on the balcony...
I completely missed the second part of this question regarding size of pots, but I see that MG has covered it.
27 Apr, 2012
Thanks Bamboo, Moongrower and Linda. The reason I ordered the plug plants is I lost all but one of my V. Bonariensis the winter of 2010/11 so I know now that I should have protected them. From the surviving one I have found only one seedling this year. Given the weird weather lately, I guess that I should treat the plugs with care as there may yet be a frost even in Devon. So I will pot them up and cover with fleece at night. Hope this will suffice.
Thanks for explaining the reason for potting on MG. I was tempted to put them straight into the ground but if they need to snuggle up in a small space then that's what they shall do.
27 Apr, 2012
Moongrower, I watched the RHS video and learnt from it although your answer about potting on explained it really well too. So thanks for the link; it made me realise that it's time I replanted a Fig which has been in the same pot for about 5 years..
Enjoy your day tomorrow.
27 Apr, 2012
Thank you Merlin - Bulba and I will both enjoy the day!
28 Apr, 2012
In the case of Verbena bonariensis, they are not fully hardy - if you planted them straight outside now, there's a chance you'd lose them. They have probably been grown under cover or inside, and need hardening off anyway, and nurseries cut them right down to induce more side shoots, all of which will be new and therefore particularly susceptible to frost.
27 Apr, 2012