??? Cordage or tinder?
Cordage that can be made from bark is usable year-round generally. Some vines as well as many seem to retain their flexability. Just be careful when it is very cold (below freezing) as the vine itself may freeze and break instead of bending. Root sources will generally be difficult to obtain due to frozen ground.
Tinders: Winter tinders are commonly available. If you are foraging for them it would be highly advisable to begin gathering as soon as you set out. Keep what you find in a warm pocket. This helps keep it dry, dries it out more and raises it's temperature making it easier to light.
Combustion occurs around 400 to 500 degrees for many materials. Some lower, some higher. If you are beginning with material that is at ambient temperature (winter time figure 0 degrees) then you need raise the temp of the tinder by a full four or five hundred degrees. If you have been carrying it in your pocket, you have already raised that temp by 70 to 90 degrees, making less heat needed to achieve combustion. When you are beginning the process with a spark or a tiny coal, that little bit of temperature difference can mean getting warm by a fire or not.
Good sources do abound. Grasses are generally covered my snow, but can be found at the base of trees. Pine cones, bark, duff, dead needles, dried seed pods, etc may be found by foraging about: look around the base of large trees where it will have been sheltered from snow and rain, under fallen trees, in dense thickets of brush. Split-wood techniques are your best friend as the inner wood and bark will generally be drier than the outer layers.
If you are knowingly setting out in winter conditions, carry your own tinder with you. Scraps of rubber tire or inner tube burn nicely. Butane lighters don't like to work in extreme cold, so pack good wooden matches in a sealed baggie and keep them warm in a pocket too. You can still forage for the experience and education, but having a known good tinder or fuel cube in your pocket serves as that all-important safety net and confidence builder.