Ok, first a few assumptions so that I may pose this question clearly.

1. Location is Eastern US, 38-40 latitude. Goal is to grow in greenhouse environment under natural light in early spring, Mar-May.
2. Mother plant is (to be) healthy and of a 45-day "trigger happy" sat-ind cross.
3. Mother is (to be) growing indoors under a 18-hour per day HID light regimen and is in a fully vegetative state at the time cuts are taken.
4. Cuts are (to be) taken on February 1, 2010. Cuts are rooted under flouresc lights then moved to a HID for modest grow-out.
5. At this point it is (to be) Mar 10, 2010, 5-6 six weeks later, and it's time to bring them out into the greenhouse (natural day-light and length) for blooming.
6. Sun is on the ascendancy and days are barely 10-12 hour total daylight, still quite short. It is 90 days until the solstice where there will be 16 hours of daylight.

Ok, here's the question as best as I can phrase it:
Without using supplemental lighting in the greenhouse to "extend" the day-length, can the young plants be counted on to go into and STAY INTO bloom, given the short days and the gradually increasing day-length of late Mar and onward into late May and June.
This question gets to the heart of whether cannabis is more sensitive to day-length (mums, pointsettias) or crop-time length (like corn). My understanding is limited. I think most of us assume it's day-length that triggers cannabis; hence the general method of moving from 18-24-hours down to 12-hours at bloom time. But can a plant that was fully vegetative under a STRONG 18-hour cycle as a young, rooting cutting be counted on to GO BLOOM and stay there in the early spring under NATURAL LIGHT ONLY as the day-length gradually increases to it's max on June 22nd?

The reason for getting this question answered clearly and accurately in advance is obvious; if this gorwing can't be done without supplemental lighting and the necessary electricity protocol and photo-period manipulation curtains (standard in the green house industry) then plans for these things must be made well in advance or greenhouse growing scrapped altogether.

Any EXPERIENCED advice and input would be GREATLY appreciated.
Peace.