Cover the beds with two inches of leaves, compost or a cover crop like crimson clover or fava beans each autumn or early spring where the climate is harsher. Turn under cover crop or leaves in spring, and plant again the next year. Light spading or forking will then create a suitable bed for planting. As time goes on, soil will keep loosening and you will be able to garden almost year round. If you are growing vegetable, do not use creosote soaked old railway ties.
Chemicals have a way to leach into the soil and thus into the plants.
The new reused snap in plastic boards are best because they will not rot or contaminate your plants and soil.