You are never going to be pleased with the production of asparagus in your garden, due to your location. Asparagus stores up energy in its storage roots for the production of next year's spears. It does this through the process of photosynthesis, which manufactures carbohydrates for the growth of the plant. Those excess carbohydrates not used in plant growth are stored for spear production. The asparagus needs bright, warm days and cool or cold nights to do this. If you have temperate growing conditions and warm nights, little excess carbohydrate is manufactured and stored. The result is declining production of fewer and smaller spears. This can be minimized to some extent by reducing the length of your harvest period to four weeks or less and allowing the ferns to develop and grow while the temperature is still somewhat cool.
You can always grow something else more productive in that space and buy your asparagus at the supermarket.
