Are you sure they're spiders and not spider mites?? Spiders shouldn't be a problem, but spider mites definately are. They tend to web on the undersides of the leaves, and make the leaves appear dry or mottled.

If you are seeing spiders they are regular garden spiders and no problem. Most garden spiders are benificial insects and eat bugs that prey on our plants.

Spider mites on the other hand do weave webs but only after plants are badly infected. They are tiny microscopic insects--You can't see with the naked eye. If these are seeds and tiny seedlings I doubt that they are spider mites. You would notice damage to the plant--tiny white spots on the leaves--before you saw the webs.

Heirloomjunkie: The webs of spidermites are from branch to branch. The spider mites live and lay their eggs on the undersides of the leaves. You can tell you have them by running a leaf between your fingers and if they are there it will feel gritty on the undersides. The reason they are so hard to get rid of is that they hatch out every 3 to 5 days.