Answer:
Belgium has no national flower, no national tree and it's anthem is La/De/Die Brabançonne. (the first three words all mean 'the', but since Belgium has three different languages the word is translated, other German name is 'Das Lied von Brabant')
People generally say that the Lilly of the Valley and the Red Poppy are Belgium's national flowers but neither of these flowers actually is.
I am not surprised that these flowers are said to be Belgian National Flowers by non-Belgian sources. If they were the Federal Government's site would make reference to them.