[grinling]

Got 4 melon seedlings for the polytunnel. When is it best to plant and what support should I be rigging up?

[hightide]

Not sure about your seedling variety, Musk is better in a heat controlled greenhouse, Cantaloupe are suitable in a cold frame or tunnel.
There are two options, if you have the space you can grow them flat or grow them up as a cordon. As a cordon a six foot run of vertical support is best, something like a 'Munty Frame' in construction could fit into a tunnel if it has the height.
If you grow them flat pinch out the growing tip after five leaves and train the four best side shoots in an X shape. Thin to the four best fruits one on each arm, pinch out the growing point and remove any new growth, feed and grow on.
If you want to grow them up a horizontal wire and vertical post construction is best, although I have used a bamboo frame; horizontal supports 12 " apart, vertical supports 15" apart. Tie the stems to the vertical support and pinch out the growing point at six foot, as the laterals develop tie them to the horizontal and pinch them out at the fifth leaf. The side shoots from these laterals carry the flowers and you should pinch out the growing point two leaves past the flower. It does need a bit of work, but you want the energy going into producing fruit not vegetation. Thin your fruits to the best four per plant, give plenty of water and feed every week. You will need to support the developing fruit as it does get heavy, I use the netting that comes from supermarket fruit packaging, before that it was the wifes cast off tights.
Planting in their final positions depends on the temperature, but you need to be above 15^oC night time.
   

[grinling]

Thanks for this, will go for flat as easier.