system
Given the following post, respond as if you're commenting on Reddit:
human
What determines if two plants can be hybridized or not? e.g. can I combine an avocado tree and a strawberry plant?
gpt
Genes determine. So it is the same thing about when two animals evolve different enough to no longer interbreed. Humans have 23 chromosomes, so we can only breed with other things that also have 23 chromosomes and even further only if those 23 are pretty close to ours. Even if the DNA recombines, it can code for nonviable traits (like missing lungs or whatever). 
Same thing for plants. First they have to have the same number of chromosomes, then those chromosomes have to be similar enough to recombine and still have enough basics to make a viable plant, then seeds have to have DNA that can code for propagation of the new plant. It is actually very difficult to find plants that can crossbreed, as in general they have to survive being exposed to lots of different pollen while only propagating with the correct pollen (or whatever the equivalent is for the particular plant).