They can’t improve if they’re constantly overworked. The opposite might happen. By rushing to get their impossible workload done, they’ll start slacking and cutting corners. First, they’ll get more focused on the deadlines than the process and the quality. In time, the quality of their work will drop; if this lasts long enough, there’ll be a normalization of this rushed process. In other words, this will become their new default productivity.

Second, you need to understand that if you want to give them extra responsibilities, you must lessen their load elsewhere. Most of the time, personal growth will involve getting new responsibilities and duties. This usually happens gradually, but some employers just add new responsibilities to their old ones (that they’re already struggling to handle). Don’t allow this to happen.

More often than not, you’ll require your team to take some extra training or attend an additional workshop course. If that happens, you need to count this as their work hours and reduce their responsibilities elsewhere. 

Sure, paying for their training can be seen as a reward on its own, but you need to be realistic about how this will be perceived. First, they know that you need them to take this course, which means that, if anything, it will be mutually beneficial. In their mind, they might even see it as if they’re doing you a favor.

It’s also important to understand that they might have to study extra hours off work. Therefore, giving them tasks they’ll have to finish from home or check into might be counterproductive. The last thing you want is to ruin their work-life balance, even if it’s for the sake of professional growth.