Can you delay your planting?

We had a mild winter, and in February I couldn’t bring myself to trim the new growth on my plants. A freak ice storm in February did not kill back the new growth as much as I might have expected, and we had early first flushes.
I now find myself facing the prospect of seedlings sprouting in September. But for the insect and disease pressures, this might not be a bad thing. (I am often visiting elderly parents during holidays, and in recent years, I have been losing very young, tender seedlings when away.) When I have tried to delay planting until after holidays, it hasn’t gone well for me either (though my experiences aren’t statistically meaningful based on the limited crosses I had those year.)
My question, I think, is probably self-evident, but how can one delay germinations, and to what extent does doing such affect the success rate, in your experience? I have in the past planted seeds from years-old dried hips from species roses, and with stellar results, but have not had luck with my planned (perhaps futile) crosses.

The bottom line is, you have to do what you have to do. If your weather allows successful germination later, then delay it if you need to. If luck finds you worthy, you may have sufficiently mature seedlings to endure your “winter”. If not, hopefully, there is always next year.

Do you only germinate your seedlings outdoors, or do you have an indoor setup (maybe with artificial lighting)? Indoor germination (with lighting, particularly) makes a big difference if you want to be able to control the timing. With that, you can let your seeds sit (dry) at room temperature and then start stratification whenever you choose to. Years of drying may not be ideal, but months won’t make any material difference, as far as I can tell. I usually give the achenes a good soak in water first (ca 8 hours) if they’re thoroughly dried; rehydration works best if you can break through the outer aril layer first (easiest while it’s still fresh and moist, but something abrasive can work after the achenes are completely dried). Dry achenes with an intact aril sometimes refuse to absorb water during the soak, but may eventually soften during stratification (although it will take even longer for moisture to work on the achene itself in that case).