Sand is good; try for something coarser like pumice or fine lava rock. Greensand is good for minerals.

Your frost date will not harm the plant but may freeze the delicate blossoms and so no fruit that year. Mine have bloomed in March.

30" rain a year is fine for seaberry.

March, huh? Sounds like trouble. I wonder if the blossoms get past the delicate stage, will a frost still kill my chances of fruit for the year?

I haven't had anything lower than 29° during or after the bloom, so I don't know.

I've not had problems with mine growing. In fact, they might be better if they didn't get the attention they get--they are growing aggressively and have roots that shoot out new growth a few feet away. I have to go out every spring (when they green up before the grass) and remove them from the yard where they've escaped the bedding areas nearby.

Fruit/flower very hard to detect, although the robins did and I didn't get to sample a single fruit this year.

Spines are for real--watch planting them where you'll back into them, or they could catch an arm or an eye.

Sea buckthorn is not effected by freezes, it grows in northern part of Russia very successfully. It it gets pretty cold out there!