To collect celery seed you need to wait until the seeds are dry and brown. Hold a bowl underneath them as you roll the seeds between your fingers. If they don’t fall out into the bowl then they aren’t ready anyway. With some practice you can hold multiple seed heads in one fingertip and twist them to make it go faster.
Using the wind I chaffed the seeds to get any detritus out. After they were clean I let them sit on a sunny windowsill for a day or so.
The seeds can be used to plant new plants or as a kitchen spice. We are going to use these for spice as they are much stronger than ground seed. The whole seed will hold its flavor longer than ground seed.
The plants we got the seeds from are two years old. I started the plants as seed outside and cut stalks as we needed. The rest of the stalks were left to grow and mature. The umbels mature at different rates so you have to keep an eye out and collect seeds as they come along. We have had several windows available to collect seed from the handful of pants that we have. Celery is self pollinating but we have several plants for good measure that we kept for seed production.
You can also cut off the top of the flowering stem and put it in a grocery bag to collect when its dry but we prefer to do it as they get dried and ready.
Store the dried seed in a sealed container and it should last for a couple years, at least being viable to make new plants. It can possibly cross with parsley so you may have to take that into consideration.
Celery seed is a good spice to use in curries and salad dressings. It is supposedly good for gout and arthritis but I haven’t tried to use them ayurvedically before so I cannot attest for their strengths in those fields. They taste good and that’s what I use them for. Its not hard to do and they will store a long time.