I am posting a link. I'll also quote the info here...

"
Propagation method

Seed collection: Tulip poplar fruit is a cone-shaped aggregate of winged seeds (samaras). Harvest the fruit in the fall after they have turned a light tan and before the seeds separate for dispersal.

Allow fruits to dry for several days and the seeds will easily separate from the fruit by pulling them apart. They can be stored dry for long periods in air tight containers in the refrigerator.

Seed dormancy: Tulip poplar has physiological dormancy.

Seed germination: Stratify seeds using moist chilling for 60-90 days to satisfy physiological dormancy. Following stratification, sow seeds in a nursery container to produce a seedling or sow them in a plastic container in the classroom to observe germination. "

Here is a link that might be useful: How to propogate a tulip poplar

I have noticed that most of the seeds I find are not viable. They just appear to be hollow samaras. Does anyone know what the typical germination rate is for this species?

Comparing the number of seeds that blow into my flower beds each fall with the number of seedlings I have to weed out in the spring and summer, I would say that it isn't really a high rate of germination. I am sorry you aren't having too much luck, but I can only be thankful for me - I would have a solid carpet of nothing BUT Liriodendron seedlings if they all sprouted!

For Tulip trees, heavy seed crops tend to compensate for low seed viability (around 5-20 percent).
The samaras are wind dispersed to distances 4 and 5 times the height of the parent tree. Seeds require a cold stratification period, and germination rates vary with time and temperature. Under controlled conditions, stratification in moist sand within a temperature range of 32° to 50° F for periods of 70 to 90 days resulted in satisfactory germination. However, seedling yield increases with increasing time of stratification. Generally as temperature decreases and time increases the germination rate increases; for example, 90 percent germination occurred after 140 days at 36 degrees Fahrenheit. Germination is epigeal and occurs when seeds remain constantly moist for several weeks. Seed Germination and seedling development is enhanced on mineral soil or on well-decomposed humus than on a thick, undecomposed litter layer. Germinating yellow-poplar seedlings need a suitable seedbed and adequate moisture to survive and become established.

After germination, several critical years follow. During this period sufficient soil moisture must be available, good drainage and protection against drying and frost heaving are necessary, and there must be no severe competition from nearby sprout growth. In a study in which various mulches were used to induce soil temperature variation, seedlings grew faster in warm soil than in cool soil. Soil temperatures as high as 97° F had a beneficial effect on seedling growth.