They look like crape myrtle seeds IMHO

They're not seeds -- and they are definitely not related to ragweed or thistle.

Rather, they are some type of fruit or seedpod: see the stem, and around it the sepals/calyx: the point-edged cup around that end of the fruit?

They look like a tomato relative like nightshade. The best possibility I found photos for is Solanum eleagnifolium (aka Silverleaf nightshade, Silver-leaf nightshade, White horse nettle, Trompillo, and Tomato weed):
info here
and photos here

Did you see anything at all like that in your planters last summer?

Other solanums found in Colorado:
S. americanum
S. dulcamara
S. heterodoxum
S. jamesii
S. interius
S. physalifolium
S. ptychanthum
S. stoloniferum
S. triflorum

S. rostratum is also in Colorado, but the photos I saw don't match.

If they are hard, they won't be solanum seed pods. By the end of winter solanum seed pods should have broken up anyway, shed their seeds and most of them are fleshy not hard.

Definitely not thistle or ragweed. Best guess is to agree about crepe myrtle seed pods. Wikipedia has some close ups of crepe myrtles including seed pods.

I need a reference. Can you put down a quarter and reshoot?

The crape myrtle seed pods I see here in WNC:

= are all smaller than the average cooked pea;

= all split open, lose their seeds, and dry up while still on the tree;

= nearly all then remain, empty of seeds, on the tree for quite a while. [As of a couple of weeks ago, the ones I noticed in the county seat were mostly still on the trees. As of 6 weeks ago, a tree near my parents still had most of its pods on the tree.]

In our climate -- which must surely be warmer than the OP's winter -- the Carolina horse nettle fruit (S. carolinense, a relative which also bears yellowish fruit, though usually well above pea-size and sometimes above marble-sized -- though it depends on the marble!) don't all disintegrate quite as quickly as Old Dutch would expect. They do, however, fall singly (with the stem attached) while still whole.

Some of the ones in the OP's photo show dents and pleats in the skin. That would be normal for drying S. carolinense fruit, but I would not expect it from crape myrtle pods.

Net result: the OP's fruit does not behave at all the way I'd expect from crape myrtle pods ... but is more believable for a Solanum ... but might be neither.

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ameast, could you cut a few of them open with a real sharp x-acto knife and post a photo, please? Be sure to include the seeds.

Do you have any crape myrtles in the immediate vicinity? (Shrubs or small trees with can't-miss-'em masses of summer flowers, usually in shades of white-pink-red-lilac.)

Just for the heck of it, do a search for slug eggs or land snail eggs. Do you have either of them in abundance in your area?

my first thought was osmacote... but i think i see plant parts attached ...

the simply question is.. what was in the box last year???? .. include weeds ...

ken

They aren't gastropod eggs because they have a visible calyx and stalk. And most slug and snail eggs are white spheres.

They look like Solanum fruit to me too. Is it possible there were voles in the beds under the snow and this is their cache?

I'm in Colorado Springs too.
Those look familiar to me, but I can't pull out a name.
Try posting on Rocky Mountain Gardening.
Someone will probably know over there.