I am a beginner at gardening myself, so this is some valuable advice I thought of today which will hopefully help other beginners. This has probably been posted somewhere, but it would make a good sticky.
This is mostly for growers who plan on spending a lot of money on equipment, such as indoor/hydro in countries with strict cannabis laws but could also be useful for outdoor growers.
1) Don't mention or even hint about illegal substances in the store. No matter how "cool" the emplyees may seem to you, even if you know they're pro-cannabis, they could be forced to turn you in or you can put that person's job or the whole business in jeopardy.
2) Always keep Rule #1 in mind, but remember that garden shop employees are NOT police and they're not there to interrogate you. So be cool but don't be suspicious.
3) Pay with cash whenever possible, especially if you have a grow spot at the same address as your credit card or checks.
4) Learn about the equipment you need. Do research on the internet or whatever you need to do to become somewhat familiar with it. Of course, you still may have to ask questions at the grow store but you don't want the employees telling you what you need if they think you're growing tomatoes.
5) Learn about other plants besides cannabis. This is not required but can be helpful in maintaining a low profile. I realized this today when I was at the garden shop - people will often come in and chat with the employees about their plants. While it's not illegal, it can be suspicious if you're always buying equipment but you never purchase or even look at seeds and plants. So today I browsed around the outdoor plants before I went in and I asked the guy about spices and peppers, and I picked up a few packs of seeds. Seeds are cheap and you might actually have fun growing something else besides cannabis.
Hope this helps everyone - there's probably more I didn't think of, but those are the basics. Feel free to add your own advice to the list.
