My sweet peas are growing quite well up the netting. As shown in the attached photo, they’ve grown up past the netting and are reaching skyward into emptiness.
(I put that black plastic square behind the plant in the upper right hand corner just to show contrast in the photo.)
If I prune that shoot so it stops growing skyward, will I kill the plant?
What should I do here, to maximize flower production?
What you should have done was pinch off the top of the first stem when it was about 4 inches (10 cm) long. That would make the plant produce more than one stem. You can still do it now, but it won't be so effective.
Just pinch out the growing point with your finger and thumb. Don't try to "prune" the stem by cutting off half of its length.
"Climbing" sweet peas like yours grow to about 6 feet (2 meters). If you wanted them just to grow up to your wooden rail, you should have chosen bush or dwarf varieties, which don't really climb, produce multiple stems, and grow to about 12 to 18 inches tall and a similar width.
Sweet peas like full sunlight, so yours might be a bit too shaded by all the woodwork surrounding them. Next year, try putting the planter outside the railings, not inside. Sweet peas are perfectly hardy, so you don't need to worry about the weather damaging them.
(It's not very clear from your picture how high above ground level the railings are, so apologies if putting the planter outside is impossible.)
Sweet peas tend to stop flowering at the point they are allowed to set seed. If the flowers are regularly cut, either newly opened as cut flowers or at the point that they wither, the plant will keep producing. Belatedly trimming a plant that has decided it's finished, in my experience won't prompt it to more flowers.
Other than alephzero's answer, you could instead force that growing tip into the netting to make it creep along the woodwork - it will probably keep veering upwards but you won't do it any harm by continually tying it down. Any side shoots that are produced by pinching it out would probably be better suited to this though.
