Cultivars do matter. Because of bloom time. Also, the flowers have to be compatable. Female flowers have receptors and need pollen from an opposing male. If the cultivars are too similar, the male flowers will kick off pollen that the female flower reads as self-pollen and fertilization won't take.

Screening shrubs and bushes. Be prepared to keep them pruned. Nuts produce on year old wood, so keeping the older wood trimmed out is good for health and necessary for getting nuts. They can also get to be somewhat big trees if left alone in a happy spot, well above double man height- and that can get to be problematic for harvesting.

I do...or rather did because once they were discovered by squirrels, it was game over because they had gotten too big to enclose in fruit cages. So, I grew a Trazel - avellana colurna (because there were a couple of neighboring street trees, even though it was meant to be self fertile. and a couple of Filberts, one a red one, name lost to time while the other was Cosford. Absolutely gorgeous nuts...and eating them fresh was a revelation to me - milky, sweet, delicious. I keep the red one but don't get nuts cos no other cultivars to cross pollinate.

I do still find seedlings popping up around the plot and am tempted to try again, with fierce protection and a rabid pruning regime...but then again, teeth (or rather lack of) stays my hand.

I have 3 of the new U of O disease resistant cultivars. beesneeds covered most of the issues to be aware of.

When they started bearing, I only got a couple years of nuts from them when my hayseed neighbors had a roaming outdoor cat (aka barn cat, but it isn't a farm). Alas, the poor guy got run over, and then those people moved away. Now, squirrels eat almost all the hazelnuts. And what they bury is starting to crop up all over as a weed. It was great though, I loved eating them.

But I'm kind of stuck with them because they now shade a bunch of rare woodland perennials and shrubs. I like them because they are small sized trees that are seemingly bred to grow as giant shrubs, so you can 'trim' the amount of shade in a given area by cutting away branches.

My 6-8 foot Americans used to make plenty of nuts. Now, they're getting shaded by my bur oak (tho I don't think that's the only issue) and barely produce any, tho they have catkins in spring and seem to look healthy enough.