You could get one of these self-fusing silicone tapes that are used to fix minor plumbing and hose leaks. You should be able to find it at any hardware store and they cost about 5$.
Now coming to using them, it's a double sided adhesive tape which can be a bit tricky at first and you'll need to do it carefully, else it won't sit tight.
- First, locate the hole in the hose. This should be very easy -- turn the water on (maybe not full blast) and pull up the hose from under the mulch. You can use little tape bits or whatever to temporarily mark the spots and then turn off the water.
- Dry the outside of the hose thoroughly and if you can unhook it from the mains, do so and blow the water out. The surface should be completely dry, without moisture.
- Now comes the hard part - wrapping it around the hose. Peel a little bit (3") and hold it firmly against the hose, about 2-3" away from the hole (or one end if it is a longish crack and not a hole). Now start wrapping it around the hose and moving to the other end till you're 2-3" away from the hole or the other end of the crack and keep peeling the clear tape as required.
- You should maintain at least 50% overlap each time you wind around the hose. Note that these things stretch quite a bit, so make sure that you pull it really well as you wind it. This is very important because the "self-fusing" nature or the reason why it sticks is because the double sided adhesive prevents it from slipping when it tries to unwind itself due to the tension from the stretching. The friction helps keep it tightly bound. If it were loose, it'll simply separate.
Be careful to not peel off more than you can wind at once. Otherwise, you'll soon realize that it gets all twisted and sticks to itself in ways that you can't get it undone.