Hi David,
" do i need to change the plants location?
now they are on a very sunny patio? "
Zinnias need full sun, which usually means at least 8 hours of direct sunshine per day.
" is it a good time to cross pollinate now? "
Yes. Now is a good time to cross pollinate. Early in the morning when the pollen florets are just opening, you can sometimes see some yellow powder in the center of the floret, and that is the most effective time to use the floret. But there is usually some pollen left in the fuzzy arms of the floret for several hours. It takes just one grain of pollen on a stigma to fertilize it. If today's pollination of a stigma is successful, the stigma will be withering tomorrow and the next day it will be obviously dead.
However, if pollination of a stigma didn't "take", the stigma will remain yellow and alive, which gives you a second or a third chance to get it pollinated. An unpollinated stigma will remain obviously alive and yellow for 10 days or more, which gives you several chances to get it pollinated. I part the petals with my fingers to examine all of the stigmas to see which ones have been "fertilized" and which ones are still alive and yellow, and I apply pollen to the fresh ones. By doing that on a daily basis you can get most of the stigmas "fertilized". You can do a better job than the bees.
As soon as the stigma is fertilized, a tiny embryo in the attached seed starts to grow. The embryo will reach maturity in about three weeks. At that time the petal will still be alive and good looking. If you wish, you can pluck the seed and plant it then, or you can dry it on a newspaper or paper towel or whatever is convenient and store it for use next year. It is rather obvious which petals have a viable green seed attached and which ones are "empty". Notice that the petals are still alive and have their color.
I don't know how long your growing season is in Jerusalem. Here in Kansas we can get a killing frost any time in October, so I am very close to my cut-off time for planting zinnia seeds outdoors. In my experience, green seeds can survive a light frost, but a really severe freeze would probably kill them.
There are several advantages to saving green seeds instead of the classic waiting until the flower head dies and becomes brown. Green seeds let you avoid pre-germination in the seed head if there is a prolonged rainy period. Green seeds reduce the risk period from seed eating birds (that is a risk here in Kansas). And, if there is still time, green seeds let you grow a second generation of zinnias in your growing season. It can be very satisfying to cross pollinate your zinnias and then see the results the same year.
If you have questions or need clarification about any of this, don't hesitate to ask.
ZM
