I was out in the garden today picking a few blooms from my Nasturtiums to decorate a salad with when I noticed how many seeds were being produced by my plants this year. When I first grew them I was worried I wouldn't be able to recognize the Nasturtium seeds and that I would somehow lose them all to garden critters. Once these annual set seeds it is pretty easy to spot them if you move the foliage aside and inspect your plants. For the most part the seeds are produced in pairs like in picture of nasturtium seeds in this post but on a few occasions I've observed them growing in groups of three or four.
When I collect Nasturtium seeds depends on how much patience I have in the garden that day. Sometimes I'll collect the seeds when they are still green if I'm worried about losing them in the garden. Other times I'll wait and lift up the foliage and retrieve the seeds that have fallen to the ground like in the second image above.
My experience with Nasturtiums seeds is that it is rare to find them perfectly ripened on the plant. I have yet to observe a brown seed still attached to the plant. If I pick the seeds when they are brown or green doesn't seem to affect germination results. Unlike other plants where the seed color is a good indicator of ripeness what seems to matter with Nasturtiums is the size. The larger the seed the better the germination results. If you're new to saving seeds from your garden for next year see my post on How to Save Seeds.
Other posts on Nasturtiums:
When I collect Nasturtium Seeds-My post with pictures and video to help you identify the seeds and when to collect them.
5 Reasons I Grow Nasturtium in My Garden-Post on the benefits of growing Nasturtium.
Nasturtium "Jewel Mix"
'Moonlight' and 'Spitfire' both are climbing nasturtiums.
Direct seed sowing nasturtiums in the garden.
Update: Made a video of me collecting seeds people who want to know when to collect Nasturtium seeds or what the Nasturtium seeds look like.