watermelon cross pollination problem

[happyprince]

Hi,
I will grow some watermelons this year and I want to save seeds.I will grow three varieties: Blacktail Mountain,Alibaba and Gerlengech (a local Turkish variety which is extremely rare.)I have just sow the seeds of Gerlengech but Blacktail and Alibaba varieties were sown at the same time and the germination happened nearly at the same time too.
I have no other place to grow these varieties.I must grow them all in the same field.
Do different varieties have different blooming time?Do they flower at the same time?I know that Blacktail is an early variety;does it bloom earlier than Alibaba? If it does,can I be sure that these varieties did not cross pollinate?
I also plan to grow some cherry tomatoes.I heard that cherry tomatoes do not cross pollinate easily.Is it true?
any help will be appreciated,
Thanks a lot in advance...

[PunkRotten]

The way I understand it is if a plant gets cross pollinated the fruit will be fine and will be true to the plant. But if you collect the seeds they will be hybrid seeds. It is OK to try and grow some plants next to eachother but if you want to save true seeds you will have to pollinate yourself and bag a few blossoms. Then collect the seeds from those bagged blossoms. Once the fruit starts to develop you can take the bags off.

[!potatoes!]

yep, you'd have to self-pollinate each variety to get true-to-type seed. it's not too complicated:
http://www.youtube.com/watch?v=q5yz4_mPSXM