There is nothing about the heat itself that could directly cause your toes to cramp up. However, one likely possibility is that the cramping is related to working outside, sweating, and becoming dehydrated. When you are outside and sweating, your body loses essential salts and water in the sweat. If you do not replace these through staying well hydrated and taking in foods to replace the lost salts, you will experience shifts in your blood chemistry that can cause your muscles to cramp. I would suggest that you focus on staying well hydrated; definitely, you should carry a water bottle with you at all times, and you should be working at keeping up with the fluids you are losing. One way to gauge this is to weigh yourself before and after a day of work; if your weight is down after working, then you are not drinking enough.
You should also be on the lookout for any signs of serious dehydration. These include feeling faint or dizzy, feeling like your heart is raising, or a clammy feeling to your skin. Also, if you notice that you are not making urine or that you have stopped sweating, these are also potential signs that you are getting seriously dehydrated and that you need to see your doctor
or go to an emergency room. Otherwise, please discuss this issue with your primary care doctor
, who can conduct a full evaluation and make a diagnosis. Good luck!