There are two ways to send a Private Message.
The first is to write a message and address it to the Member, the second is to go to the members 'My Profile' page and send it from there. In either case you'll need to be a member of the Mulch (it's free and easy), and be logged in.
The first way is just like writing an email. Start by going to the Private Message page, to do this, hover your mouse over the 'My Profile' navigation, then the 'Things to do' link then click on 'Send a Private Message'. - you'll be taken a page where you can write your message, check for incoming emails or see old one's you've sent.
To send the message you'll need to choose who you'd like the message to go to, do this by clicking on "Show Users" link. You'll see a list of every member in the Community, click on who you'd like to send the message to (it can be more than one person) then click 'Add Users'. Then click 'Send' and whoosh - off it goes, just waiting to be answered by one of your gardening friends!
The second way is to simply go to the Member Profile page of the person you'd like to send the private message. Once on their profile page you'll see an area in the upper left hand corner to write them a message.
Note: Anytime you see a members username of Avatar (their profile photo) you can click on them and be sent directly to the members 'My Profile' page. So if a member asks a question in the forum, or posts a cool photo, or has a similar plant as you (or for any other reason), you can easily send them a message!