We will start off with a blank canvas, and making sure our colors are default (black and white), we will goto Filter > Render > Clouds.
Next, we will goto Filter > Blur > Motion Blur and set the angle to 90 and the distance all the way up.
If the difference between the light parts of the image and the dark parts is too great, we will want to goto image > Adjustments > Levels. And change it to something like this:
The first filter we want to use is Filter > Brush Strokes > Accented Edges. Play with the settings and find something that looks good.
Then we will goto Filters > Artistic > Dry Brush. Again, experiment with the values.
Finally, we should add a tiny amount of Filter > Artistic > Film Grain.
Already, our wood texture is almost there. We can add some color to it by unlocking the layer (if it's still Background), and then going to Image > Adjustments > Hue/Saturation. Make sure the Colorize option is checked.
Now we want to goto Filter > Liquify. We can use the Turbulence tool on the left to somewhat smudge and distort the wood grain:
Then we can use the Twirl tool in Liquify to recreate the knots:
As with everything in PhotoShop, experimentation is definitely the key. Try changing values around to see what outcomes you get.
More Tutorials: 
