What if humans’ interference with nature stops plants growing entirely?

No flower, no trees, not crops, not even weeds. The effect of pollutants and pesticides, overproduction of food, etc. Plants can still be grown but it has to be done manually and takes a lot of work. Therefore, giving someone cut flowers isn't so much just a small nicety as being more akin to diamond jewellery or showing off wealth. The fact that you can afford cut flowers indicates an excess of disposable income.

Food is still produced but it's entirely synthetic with rare exceptions. A fresh tomato is akin to caviar.

Trees are usually synthetic and decorative because of the difficulty of maintaining.