Try Carol Depp's book " Breeding Your Own Vegetables" for instructions on cross breeding . Corn, cukes, melons, squash, pumpkins and sunflowers are easiest to cross breed.Try favas soybeans of the North for beans ( actually a vetch ) . Cukes and melons can sometimes be crossed !
check out Ken Whealy's "vegetable seed inventory" or the "seed savers exchange" to locate breeding stock. good luck !

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I think that daylilies are also easy to hybridize but to get good results it may take a year. How long do you have until the science fair, that would help us give you suggestions.

So now that you have had time to request the books from interlibrary loan decide what you want to crossbreed .
A few years ago a science fair kid was going to cross a zucinni squash with a big max pumpkin ( actually a "squashkin") you may wish to work with radishes there are many types and colors they sometimes have a quick maturity . Or you may wish to work with lima beans crossing the Fat "Greek giant limas " with the Doc Mertins or whatever the largest type is or cross either or both with the earliest maturing type (Henderson's) . Or with corn you can cross strawberry popcorn which has up to 36 rows of kernels with the largest giant kernal variety from Perue (this type takes a full 6 months to maturity) (I could send you some corn seeds if you wish) to see what you get . Spinach goes to seed quick probably in 40 to 50 days if grown in the summer . Check with your county extension workers to see what they reccomend ! So dig up your garden plot and get busy!

thank y'all soo much this really has helped me alot...
i will post the result asap...

check out the cabbage family you can crossbreed : kholrabi , cabbage ,red cabbage , collards ,mustard, rutabaga, turnip ,rape, broccoli, cauliflower , bok choy ,kale,all types of cabbage , kolhbaga . with each other . You can cross collards with cabbage to make a quicker cooking collard or a less smelling cabbage . Or cross one with one of the giant breeds to get a giant size version of something .

You may want to cross the GIANT HEAD sunflowers with the colored petal types or the "teddy bear" type the goal is to get the giant size ant the colored petals . white Italiain , orange , light yellow, wine red , red ,brown and yellow are available . good luck.

It all takes time, sweet peas would probably take 3 months at least, sunflowers even longer, can you maybe get your hands on some fast plants, I'm not sure where you would get them or even what the binomial nomenclature for them is but they take only a couple of weeks to complete there life cycle, they were used for nasa experiments and are not uncommon in middle and high school biology and botany classes.

Here is a link that might be useful: http://en.wikipedia.org/wiki/Wisconsin_Fast_Plant

You may want to try "Hopi pink corn" (70 days) it does best in all drought tests and is sold by native seeds search of Tucson AZ . You might want to try some and breed some in to your strains.

You can cross it with other stains of corn such as "Sweet Nothings" (50 days)sold by "Peaceful Valley Farm Supply P.O.BOX 2209 , Grass Valley , California 95945 , to develope a drought resistant short season corn .

what did you decide on ? and how did you make out?