Hi Deborah,
This is a great question and it’s wonderful to hear that you are growing some oats!
There are the two (or 3) stages/parts that you can harvest – the ‘milky oats’ and the oatstraw.
For timing generally, you want to wait and look out for when the oat seeds start showing up. When you start to see the seed heads develop, use your finger and thumb to gently squeeze it to see if any ‘milk’ is coming out. Often you’ll have a ‘tower’ of oat seeds on the stem and they will range in milkiness, harvest them when the most of them are fresh with ‘milk’. You can strip these milky oat seeds off to use and then and cut off the top bit of the seed stems that are left (I haven’t used the seed stems before personally, I put it in the compost). You can dry the milky oats, or make fresh milky oat tincture, etc.
The oatstraw is the green leafier parts below those oat seed stems. This is healthy and happy to be harvested up to and during the milky oat stage. It can be harvested after the oats as well, just keep an eye on how vital it looks really. If it is yellowing, browning or becoming mouldy it is no longer of use as medicine.
Also the oats themselves still have the similar medicinal value that both milky oats and oatstraw has. All three have slightly different energetics and strength, but all are nourishing to the nervous system.
I hope you have a wonderful time harvesting oats and that this was of some help,
Benna