You can reply to them using your Inbox, its the little paper airplane icon in the bottom right hand corner of the website. you can message people their by searching their name. Or alternatively by go on their profile and hitting chat it the upper right hand part of the web page.
growdiaries.com/grower/hashy
It used to allow you to reply to your own questions but that option was removed because some morons threw a fit. so the option is no longer there :(