Why does Canada recommend a minimum rate of climb for ultralights of 300 ft/min? What is a safe minimum rate of climb for ultralights? I understand most ultralights climb at least 300-800 ft/min and in fact the Canadian ultralight specs recommend a minimum rate of climb of 300 ft/min. Even the Lazair's specs note a rate of climb of 400 ft/min.

Why does Canada recommend 300 ft/min specifically? Why not 120 ft/min?  60 ft/min? Even at 60 ft/min, you could clear a 4' fence in 4 seconds. Of course you would want some clearance, so say 20 seconds. Seems pretty good in my book. Need to clear 50' trees? Just kite around in a circle.
When you fly gliders you discover it's quite common to run into air that's descending at 1-200 fpm, or "sink" in soaring-talk.  Descending air next to a thermal, or air descending due to downsloping terrain.  It's a lot more than that at times, but a couple hundred fpm is typical.  

On a day where there's any convection (with rising air, there is always equivalent descending air adjacent to it) you are in sink quite a lot.  A 300 fpm ROC provides a reasonable assurance that you will be still able to climb while in most (but certainly not all) conditions of descending air, barely.

If you have an ultralight that can only climb 100 fpm, this might be fine on a smooth day with stable air, if you're patient.  If there is any vertical motion in the air however, you have a problem.  The minimum climb rate requirement is an attempt by the regulator to ensure that that average ultralight buyer will have a machine that won't kill him because it couldn't even out-climb a bit of subsiding air at the end of the runway.