I can't help you with the answer. But I just bought two. I am going to try and do zinnias in the method you describe. But I'm going to wait until April to start them. I will try and remember to post back my results. Again, sorry I can't help you now.

It's a container of types, but you don't have vents in the top to let in rain or water and let out over-warmed air. You're going to be doing a lot of watering so I suggest using broad containers that will have a depth of at least three or more inches of soil.

Yes, it worked for me in our cool wet spring in WA state.

I put it on raised bricks set out in the driveway on the north east side, so got morning sun then again filtered sun after 3pm. This year put it away from the house to avoid so much midday shade. Still there is dappled light some parts of the day due to tall trees.

I did what others suggested for tomato seed sowing & it worked great. No slugs either!

I used clear plastic totes (sweater box & larger sizes) on the shelves to hold plastic cups saved from coffee drinks at least 16 oz or larger. I drilled holes through bottoms of several cups at a time.

Quite a few fit, so I labeled totes with masking tape & number of cups that fit. I grouped slicers together & plum together.

In my cool, wet spring I didn't have to water until nice days in May, then simply poured lukewarm water in the bottom of the totes to soak up from the bottom. After soil darkened again or up to an hour I drained the water out most of the way.

Zippered shut for the 1st few weeks in March, then vented with 1/2 way unzipped until they germinated when I moved unit to full sun.

Then I kept both zippers open (rolled up & tied) if days were over 50. Zipped back up at night or tossed a sheet over the entire thing to remove in the morning.

You'll have to adjust what works yourself with zippered or not and watch the wind on it. Fortunately the legs fit easily between additional bricks, so we had no trouble as long as I tied up the flaps or kept them mostly zipped.

I've used clear packing tape to fix rips in top from age. Also sewed next to the zipper where it meets the plastic. Be sure to carefully hold the sides when zipping up or down to avoid pressure there. Mine has just 3 shelves & was purchased used inexpensively & works fine with the extra venting. I did add some string inside for cross bracing to make sure it didn't flex too much in wind since it wobbles a bit.

Corrine