New planting

I think I planted a hydrangea too shallow. Should I start over, and plant deeper, or what? Also can soil around newly planted plants be continually amended or is there no need or way to do that?

If you just planted it, a few days ago to a few weeks ago there is no problem in GENTLY digging it up and setting it a bit lower. Most plants should be placed a bit higher than they were in the original container, because the soil in the hole often subsides, so the plant sinks to where it should be. It is possible to overdo this, and expose too much of the root ball. The alternatives are to add more soil around it, then mulch so the soil will not erode or to replant.
If the plant has been in the ground for a while, and it still growing just fine I would not disturb it. Mulch it.

If the soil around the plant is properly amended to begin with that should be enough for the first year, but each year it is possible to add more soil conditioner on top of all the planter beds and dig it in. Be very careful doing this, though, some plants are quite shallow rooted and you would do too much damage digging the soil like this.
A simpler way is to keep all the exposed soil mulched and let the earthworms cultivate it for you.

I really feel like I almost always see (hydrangeas especially) root ball exposed when I buy plants at places like lowes, home depot, or walmart. I always try to loosen the rootball before planting (if possible) and top dress with a little good new potting soil or manure. :) they seem to like this. also because of the layering habit of hydrangeas I am not sure that you really can pot it too low. the root may suffer a bit if you did, for lack of air, but I am not sure. they are pretty hardy once they are settled. I also highly recomend if you do dig a deeper hole make it wider too! you want your new roots to really flourish and if the plant has already flowered you will want to remove any spent flowers to focus on root development. :) i like to add bonemeal, manure, and potting soil whenever i plant anything. i have also heard that lab blocks for rabbits and rats make a great fertalizer. and i have used itin holes as well! i hope this helps!

I would whole heartedly agree with Outlaw and Diane, If the plant has been in the soil a few short weeks then no harm will come to it by lifting it out the soil and replanting to the instruction as give by the Outlaw,
As you sound new to gardening May I add the advice re adding any animal manures is to ALWAYS make sure these manures are well rotted, no smell and crumble in your hands, that way you know it is ready to be used for plants.

I was always told to plant SHRUBS & TREE/S to the same depth that they were growing in their pots, when removed from the pot, you will see the darker marking on the stem's / trunks and that is the mark where the soil was the lighter area is where the trunk / stems were above the soil so a garden cane laid across the dug out hole and that gives the setting for the shrub to be planted level with, as you add ferlilizer's or manures, always add a handful to the soil laid aside to backfill the hole, as you LOOSEN the roots don't tug or pull them, you really only need tease them free a little so they don't grow around in circles as in the pot.out. to find out if the plant needs water, stick your fore finger into the soil and if it's dry, water if damp / wet, it does'nt want water.

You will have a stunning plant next year with all the TLC given, this year a newly planted Plant normally likes to work on settling into it's new environment and making roots so try not to worry about huge flowers ect.
Good luck and best regards.
WeeNel.