The first thing I would like to note is that you must be careful with using chores as a punishment. You don't want your child associating work with bad behavior. It could become an ingrained subconcious thought that work = bad. This could cause problems later when they are old enough to enter the workforce or in just maintaining a clean home when they have their own.
With that being said, the weeding issue may require you to sit out there with him the entire time. You can grab a book and a lawn chair and catch up on some light reading while keeping an eye on him. He can sit in front of the garden staring at it until it is done. If he tries to get up and move around the yard, pick him up, put him back down in front of the garden, and remind him that he can get up and play when he is done. Also remind him constantly why he has to do the weeding.
For example, "I understand you want to play, but you made the choice to do ...., so you must clean out the garden as punishment. You can go play when you are done."
Make sure he knows that what he is doing is a consequence of HIS CHOICES. It can become repetitious, tedious, and boring, but you will just have to keep repeating yourself over and over.
Time outs in the corner have never worked for my children. A punishment that I have found works for my children is squats. Make him stand against the wall and slide down the wall slightly, so the knees are slightly bent and the back is against the wall.
You can also make him stand in the corner straight up holding his arms out, like he is going to be doing arm circles. If needed, you can make him hold a tuna can in each hand that he has to hold up.
You can combine these two by making him squat and hold his arms out.
With both of the above punishments, the timer starts once he is in the position. Typically 3 minutes holding the position, in the beginning, is enough.
My son has had days where nothing worked with him. These are the days where he has become biligerant, stubborn, and even physically violent. On those days, I had to go to drastic measures to get through to him. I will send him to his room and remove EVERYTHING. No toys, no gaming systems, no TV, no books, NOTHING, except a bed. Its like a mini boot camp. He is only allowed out of his room to eat and go to the bathroom, but must ask permission before he can leave. This reinforces that he is being punished and cannot roam about freely.
After the allotted time, sit down and talk to him, face-to-face, about why he was stuck in his room and why he lost his stuff. Do NOT give him his toys back after he is released from his room. Make him EARN them back through good behavior.