how are seed companies able to know the sex of the seeds they sell?

i bet this is a stupid question but how are seed companies able to sell 100% female seeds

They stress a female or hermi to turn male. They then use the pollen from that to fertilize a female. All seeds from that are sposed to be female.

how would you stress a female and how much should it be stressed to convert it to a hermi/male

stronic said:

how would you stress a female and how much should it be stressed to convert it to a hermi/male

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Two commonly used foliar sprays are gibberellic acid and colloidal silver. Another method is light poisoning, or interrupting the 12 hour darkness period required to induce flowering.

Different plants need different amounts of stress to hermi. Some do it when you look at 'em funny, others will take quite a bit of abuse.