I live in NC, and even though we have very mild winters, it still drops WAY below 60 degrees.
I have been really reading up on heartworm, and I don't want to give my dog poison when there is no possibility that they are around.

Cathy
when she first came home

I follow a similar schedule for heartworm prevention for my dogs.
We live in southwestern Ohio and by the Mastiff site's map I would start dosing in June and give thru November. What I do is note on the calendar the fist day I see a mosquito (usually in late March to early April) and make a note to start giving heartworm meds 30 days later. Then I note on the calendar to give another dose every 6 weeks until 30 days AFTER the last mosquito of the season (usually they are gone by late November).
This schedule will be altered if the dogs travel far for shows, like the time I went to Florida for Nationals in November and another year to shows in Florida in December. Next week I'm taking 1 dog to NC for shows, so her schedule will be moved ahead.
Also I use the liquid Ivermectin for prevention and can tailor the dose to each dogs' weight more closely than the pre-made pills or chews. Those are set up for 2-10#, 10-25#, 25-50# and 50# and up, where with the Ivermectin, I can go in about 5# increments.
But everyone's situation is different and each person has to go with what is comfortable for them.

and that the disease is serious enough to try every reasonable prevention and this includes yearround protection rather than trying to 'cut corners' to save possible drug overload. The medication is effective, not that harsh, also prevents some internal pests such as roundworms and it should be less stressful for the animals's system to remain on a medication than to stop/start it. In addition, if your vet knew you were doing this, he/she would require you to retest your dog each Spring before you resumed use since giving the medication to an already-sick animal is deemed 'harmful'.