Envy is a sin, I know, but I have developed a bad case of poly-tunnel envy over the past few weeks.

These polythene palaces are warmer than my unheated greenhouse, a fact that I am aware of in early spring when I hand over my trays of root-trainers filled with sweet corn seed together with tomato seed for Marion to grow for me in her poly-tunnel.

Lately though, I have also seen Julia and Russell’s poly-tunnel that is not only stuffed with vegetables and such rarities as oyster mushrooms but has the addition of a stable door in place of the usual plastic entry and so I have stable door envy too.

My visit yesterday to take a picture of Joy happily snipping stray shoots from her grapevine had my eyes flashing as green as the vine leaves.

She planted the grapevine which runs from one end of the poly-tunnel to the other around 12 years ago. As it was a gift seedling Joy has no idea of the variety but she does know that it produces deliciously sweet grapes.

Not only are they sweeter than the grapes that hang in black clusters from the grapevine that forms a dense canopy over my pergola but as they are grown under cover they are also thin skinned.

Like my vine and every other grapevine come to that, it grew quickly and were it not pruned back to the central rod every year it would quickly cover the entire poly-tunnel because every vine has in its DNA a desire for world dominance.

Joy keeps hers in check by pruning back the fruit spurs to one or two buds on New Year’s Day as, traditionally, it should be pruned before Epiphany.

I know to my cost that should I forget and then try to cut back in early spring the vine will reproach me by bleeding sap for days. Our methods only differ in the severity of our pruning. I cut back in winter to maintain a framework over the pergola whereas Joy cuts back to keep short laterals all along the length of the central rod that was originally trained up and then along the roof.

We both cut back new green shoots when they grow too long to maintain our framework and when the vine flowers and sets fruit we snip of any leaves that are shading the grapes.

Now that our vines are mature I leave all the grape bunches and Joy thins sparingly. We could, if we chose, thin the individual bunches to produce bigger grapes, a fiddly job which neither of us bother with.

It’s thanks to the Victorians that grapevines gained a reputation for being tender plants needing a greenhouse although it was also common practice to plant the vine outside and introduce the stem through a hole in the greenhouse foundation or wall.

Grapevines are hardy although desert varieties such as ‘Lakemont’ ripen best under glass whereas ‘Dornfelder’ ripens just as well outside.

Still I have my eyes on ‘Black Hamburg’ planted at Hampton Court by Capability Brown, now all I need is a poly-tunnel for this famous grape.