I'm planning on beginning a phd in plant sciences/plant biology/plant genetics and breeding (not sure which exactly and they kind of overlap), and am wondering whether it would be better to undertake it in the US or in the UK.
I'll have a US undergraduate dual degree in biochem and plant science, and four years of research experience (including a thesis) and I am from the US.
However, I am hoping to secure a postdoc position in the UK after I finish my phd.
Does anyone have any advice?
Thanks :)

I don't think it will make any difference. Plant science is strong in UK and US. People won't care where you got your PhD from as long it's from a good university with strong plant science research.

Just be aware that a PhD in the UK is a bit different - usually no taught parts so pure research for 3-4 years. Unless you have one of the newer style ones with one year of taught first but these are still rare in plant/animal sciences. But there shouldn't be any difference in how people view it afterwards.