When To Flush

Hi all,I'm 26 days into flowering and the pistills on my plants are just starting to turn an off white colour and becoming a little withery.What I'd like to know is when's the best time to flush the plants with pure clean water.I was gonna wait another week until the pistills go a little darker or can anyone give me some correct advice on whens the best time to do it and is it absolutely neccessary(I know it produces a nicer smoke but it doesn't affect the bud yield?).
Now,this may sound like a real stupid question but i am a newbie.Can the growing medium be reused or is it best to discard it and start afresh?

You need to describe the system. What is the growing medium, and are you talking about flushing a dwc reservoir?

hes using soil...at least thats what I see...

Can't really tell whats going on with your plants as the pictures are all washed out by the HPS...take some more pictures of the plants when the lights are out...you only need to flush to remove excess salt buildup in the soil...but we dont know if you are overnuting your plant or undernuting it based on the pictures

many growers reuse their soil for about a year. amending it and enriching it for each successive grow.