I have a white berry that is in the fourth week of flower, and it is almost to the top of my tent already. It also seems to be flowering at a slower rate than the G13 next to it. Can anybody help me? What do I do?

top it or tie it....it's done growing vegatatively so get her to where you want her. at this point it wont hurt the plant. but dont wait much longer.
have you grown white berry before?

LST it! You can tie it down and it will grow vertical instead! No worries just tie her down and she'll be good to go

I have never tried the white berry, and I have already tied it down. For some reason, she kept going after I sent them into flower. She kept getting closer and closer to my light until I came home this weekend and one top was touching the light.

These plants are'nt very smart about not touching hot shit, they seem to go back for more during the first 3-4 weeks of the flower cycle. The flowering stretch is completely normal for marijuana. They always stretch to at least double and sometimes even tripple the height they were when you switched to light schedule to 12/12, no worries it happens to all of us. At this age into flowering you have done right by her by tieing her down. She's probably done stretching by now and all that energy will focus on flower development.

The White Berry is twice the size of the G13, and it seems to be developing flowers slower than the G13 as well. Is there any explanation to this? I am worried that I will only have one good one!

If I'm not mistaken G13 is an indica and will fruit earlier than sativa. Is you white berry a mix of sativa and indica, my guess it is both. With that in mind it explains the differance you are seeing. G13 has been around for a long time so it is a very consistant and sturdy strain where as the white berry could be an f2 hybrid and it hasn't come into full perfection yet. Hope this answers that burning question.

them a still grow vertically during flower is wat people need to realize.,.,tie her up and shell focus on side sho0t and bud growth.,.,is it sativa?

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I'm pretty sure White Berry is indica dominant.

Perhaps indica dominant but as along as it has some sativa to it chances are that genetic trait will show itself. Even recessive traits show up once in a while.