Yes, do row covers block sunlight has a clear answer for you. They do block some light but only a small part depending on the weight of the cover you pick. Light covers let through almost all the sun while heavy covers cut more than half. The light transmission row cover value is what you want to check before you buy.
I have grown spinach, lettuce, and arugula under light covers for four seasons in a row now. The covered greens grow at nearly the same rate as my open beds with no real loss in size or color. I tested this side by side last spring with two beds of lettuce. The covered bed gave me two pounds of leaves while the open bed gave me two and a half pounds over six weeks of cuts.
The fabric does not block light like a solid sheet would on a sunny day. Spun-bonded fibers scatter and bend the rays as they pass through the cover. Your plants get diffuse light rather than direct beams from above. This soft light can be a plus on hot summer days when full sun would burn tender leaves on greens or seedlings.
Cover weight is the main factor that sets the row cover percent light you get below the fabric. Per Maryland Extension, lightweight 0.45 ounce covers transmit 90 to 95% of sun. Medium covers at 0.55 ounce let through 70 to 85%. Heavyweight frost blankets at 1.5 ounce cut light to just 30 to 50% of full sun.
These numbers matter for the type of crop you grow under the cover. Most veggies need at least 70% sun to grow well and set fruit. Light and medium covers fit this need with room to spare. Heavy covers only work for short use during frost nights when sun is not a worry for those few hours of cold air.
Pick a light cover for full-season use on greens, herbs, and root crops. Medium fabric works well for transplants in spring or pest control during the warm months. I keep both weights on hand at all times so I can swap them based on what I need that week. Match the cover to the job for the best results in your beds.
Shade under row cover is rarely a real problem with light grades on most crops in your garden. The only time you get too much shade is when you leave a heavy cover on past the frost window. I made this slip my first fall and my kale turned pale yellow under a heavy blanket left on for two weeks in mild weather without need.
Reserve heavyweight cloth only for overnight frost or hard freeze events on your beds. Pull it off in the morning once temps rise above 35°F so plants get full sun. Use lighter weights for any day-and-night cover during the warm months. Match the weight to your need and your plants will thrive with no shade stress at all.
Read the full article: Row Cover Garden Guide: Weights and Timing
