Penstemon campanulatus

Most seeds require oxygen to germinate, if buried too deep in their growing medium, or if the medium is too wet, the seeds may not get the oxygen they require.
Some seeds need to be in the light (surface sown) or in the dark (sown deep enough to receive little or no light) to germinate. A rule of thumb is to cover the seeds their own width deep in the growing medium, but some seeds prefer to be sown much deeper, and some fairly large seeds like to be surface sown (or higher).
Many seeds germinate best at certain temperatures, some will germinate at a comparatively wide range of temperatures, yet others need fluctuating temperatures.
Almost all seeds are waiting in a dormant state for some outside stimulus to break their dormancy, some just need sufficiently high ambiant humidity, others need scarification, vernalization or to be passed through the intestines of an animal.

Penstemon campanulatus seeds will usually germinate in 18-21 days.
Normally will only germinate with light so surface sow. Sow Penstemon campanulatus seeds about 1mm deep in a Well drained seed sowing mix at about 15°C.

Penstemon campanulatus seeds need to be "overwintered" before they will germinate.

Stratification; cold treatment or vernalization. Seeds of some species need just a couple of weeks, others 3 months. Seeds can be stratified in dampened peat or sand, in a plastic box or bag at 4°C or 5°C in a refrigerator. The seeds should not be frozen or in a wet medium. Very small seeds can be sown on the surface of their growing medium, in pots sealed in plastic bags, and kept in the 'fridge. Many vernalized seeds need light to germinate when they are sown in the "Spring".
Seed Germination Theory and Practice by Professor Norman C. Deno