It's true that this plant is the type that only lives 12 months or less. This is called and "annual" type of plant. Those that live longer are called "perennials." The leaves will turn yellow late in flower as the plants die after large bud production. We really need to see more of the plants and we also need to know if you're giving fertilizer every time you water. I'm guessing that you are. Don't just look at the yellowing, but also look at the other leaves. These appear dark green which usually means over ferting. At the risk of using my "go to" explanation for why your leaves are dying and getting criticized for it, I'm going to say you have ferted too much over time. You don't want to over fert in soil as the fert tends to stay in soil, so you have to take be careful how much you give in one shot and how often you feed them. If you're giving fert every time you water, I'd say you've over done it. Even if you don't fert every watering, I'm still going to say you've over done it because you have dark green leaves and it's just a matter of time before the rest start turning yellow. One thing we need to see is the bud production. If you have small bud production despite 5 weeks of flower, it's probably because you've over done the ferts and made your soil pH too low as a result. So more pics are needed and also whether you fert with every watering. I'm willing to say I could be wrong, but based on what you've given so far this is how it looks.
