If by seaweed, you mean seaweed extract products, They usually have a limited shelf life, beyond which some of the active ingredients (natural plant growth hormones) may no longer be useful. On the other hand, the trace elements and nutrient components will still be useful.
The packaging should have an "expiry date". Make sure that your purchase is dated beyond when you expect to use the product, especially if you want the benefits of the plant hormones. If you need the product to have active plant hormones in July, a product that expires in May will disappoint you.