Using day length or latitude to estimate a harvest date

So I'm doing a guerrilla grow somewhere I can't easily visit regularly, so I can't really decide when to harvest based on appearance of the plant -- I need to pick a date and just plan to harvest then whether it's the perfect harvest date or not. I plan to only trek out there when the NOAA 14-day precipitation map shows it's been dry, which will hopefully be only once or twice between now and harvest. Also, the average first frost date for the area is mid-October, so I will need to be pretty closely monitoring weather conditions around then and it would be great to have an idea in my mind of how far along it is to weigh against the danger of a cold snap when deciding whether to pull them early.

The plants are clones of a Dinafem Super Silver lady I've grown indoors, and my indoor one was nowhere near ready to harvest after 70 days of 12/12, but unfortunately I had to harvest it for the same reason I had to get ride of these outdoor clones by planting them in the wilderness -- I'm moving unexpectedly and I need to prepare my house for being inspected by the landlord. Since I didn't let it finish, I don't know how many days should flower for, but from what I've read from other growers it often goes 100+ days, but is ready enough after 80 days or so.

So, what day length actually tends to trigger flowering to start? About 13 hours of daylight? 12.5? I've found charts showing day length, so I imagine it'll be fairly trivial to determine a plausible harvest date.

Also, I know that Dinafem's official page for the strain says late October for outdoors, but I imagine that's pretty dependent on latitude, and I don't know if they're using their own latitude (Spain somewhere?) or some arbitrary one. They also low-ball the flowering time (no way this thing would be done after 70 days), so I wonder if they're lowballing the outdoor harvest date too.

Any thoughts?

Dude.. I think you are over thinking some shit and not thinking other shit.. if your in the US generally flowering starts in august. Also, if its really wet, your buds are at risk for mold, mildew, and rot. You should be shaking the water off of them after every rain. And 80 days is usually pretty good for most strains.

I appreciate what you're saying, but I doubt mold will be an issue at this location. I also realize that flowering begins sometime in August usually, but that's a 31-day window. If I can't check on the plants more than once or twice between now and October, I'd like to still get a decent idea of when they'll be finished. If they start Aug 1, 80 days would be around Oct 20th. If they start around Aug 31, they won't finish until closer to Thanksgiving. That's a pretty big window.

If you're telling me there's no reliable way to predict when flowering will begin based on day length, I say poppycock. Day length is the only reliable way, I'd say, and using the month of August as an estimate is way too imprecise.

First off, how many hours of light are they getting now? If you are giving them 24 hr light, then dropping it down to 14 might trigger flowering. On the other hand I have several plants outdoors now and all are at different stages of flowering, even thought they were all planted at the same time. I live in an area where the daylight hours don't change more than 90 minutes or so. I vegged them on a 24 hour cycle for 4 weeks before they were moved outside. So anyway flowering is a little unpredictable, but you will probably be going out to check them at some point, and you will probably be able to determine how much longer they have at that point. Hopefully they will all be ready at the same time. Good luck.

Obviously it's strain dependent, but depending where in the states you are as well. At least I'm Cali, flowering starts anywhere from mid/late july to early August. Aside from heavily sativa dominant strains, most are good to go first few weeks of October. Unless you are way south or north, I'd figure early October as the best time to harvest

I would not expect to go past Halloween.

Thanks for all the replies! I think I'm going to just do as cindysid suggests and hope I can gauge how close they are one of the times I check on them, and I'm guessing that it won't really be quite finished by the time the first frost sets in, so I'll just have to harvest then anyhow. That being said, I figure I should beat this dead horse to death one more time...

I already know that this (90% sativa-dominant) strain will need to flower for at a bare minimum 80 days, but potentially closer to 100 days. Just to give an example of the day length variation within the U.S., the following shows the day length on August 1 and August 31 for an example northern and southern city:

Duluth, MN:
Aug. 1: 14h 52m
Aug. 31: 13h 24m

Key West, FL
Aug. 1: 13h 16m
Aug. 31: 12h 39m

Obviously if you plant two clones of the same plant in MN and FL, the MN one will start flowering quite a bit later than one in FL, right? So the FL one will probably finish flowering as much as a month or month-and-a-half earlier than the MN one, right?

And to answer cindysid's question, I already planted these two clones outdoors over 2 weeks ago. Prior to that, one was on an 18/6 schedule and the other was 24/0. I probably will check on them in about a month to make sure they haven't been eaten by rock-climbing deer (or feral goats?), but I have been expecting they will only just be starting flowering at that point, if at all. I'll probably plan one more visit a month or so later, with the hope that I'll be able to gauge their flowering stage at that time.

Mohican said:

I would not expect to go past Halloween.

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Well, it is a sativa-dominant strain, and some people report it going 100 days or longer. If it flowers for 100 days and finishes by Halloween, that would mean it will start flowering in 10 days, which I would guess is unlikely with about 14.5-hour days right now.

obijohn said:

Aside from heavily sativa dominant strains, most are good to go first few weeks of October. Unless you are way south or north, I'd figure early October as the best time to harvest.

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As I mentioned, this is a heavily sativa-dominant strain, and I am at a latitude similar to far-northern CA or southern OR, so a little on the north side of yourself. With the combination of those factors, I'm going to guess it will probably be harvestable by Halloween-ish, but won't really be done until mid-November or later. So basically I'll probably have to let the date of the first hard frost determine my harvest.

Thanks for bearing with my brainstorming, everyone, I'll report back with flowering details so the growers of the future can have an idea about this strain at this latitude.