You can grow pretty much anything under the hoops in spring. It does grow faster inside than outside. Sometimes I sow the same thing (like spinach and radishes) under the hoops and in the open ground at the same time. They grow at the different speed and are ready at the different times. In the sunny days the temperatures can easily reach 90 degrees even if the outside temp is in 50s so the care should be taken to monitor the weather every day and to open the tunnels for ventilation or to close them at night. Also watering on the regular basis needs to be done. Tomatoes also can be taken outside and grown under the hoops. In general they like the higher temp and they grow faster. They survive the frost fine, but the leaves touching the plastic may be damaged. Unless the tomatoes are very young the care should be taken to protect the plants from the bright light for the several days after putting them in the tunnel.

Watch the temperature. I find it very handy to have a remote thermometer so I can see the temp in my high tunnel while standing in my kitchen. I'm in NH zone 5 and some sunny days it gets to be in the 90s if left closed. I have 6mm greenhouse plastic. The Eliot Coleman books have great advice.

I was surprised how well my tomatoes and peppers did under plastic. Even with the sides rolled up and doors open it still easily got in the 90s or slightly over 100. But my tomatoes grew a lot better and looked a lot healthier than the ones outside that got rained on. (Tried both black plastic and grass clipping mulches last year).

I seem to do better at the end of the season. I use Agribon covers over bush beans to protect them from a light frost we usually get mid-September. If I can get the beans to survive one or two cold nights, we can get a nice crop late in the season.

I'm an advocate of plastic or row cover technology but expect about one zone of gain for a normal layer of these. You therefore would want to plant accordingly. In zone 4 you probably would still not want to put tomato plants out for another 3-4 weeks. Even then you need to be more proactive if you have night temps in the lower 20s or bright,, sunny days. Often plastic is slitted to allow hot air to escape.

More Hardy crops are a better option for early plantings. As Glib mentioned collards are very hardy. I mowed off my collard crop that overwintered without cover and all plants were loaded with lots of renewed growth. I just doubt that that crop would be profitable enough to justify covering costs for most growers.

I at one point cooked plants down to soil level when the end flaps that I had left open blew closed on a windy day. So the comments about watching temperatures are right on the money.

Post a pic of your hoop house!

Thanks for all the tips!

Here's a photo of the hoops before I put the plastic on. My garden is 24x24 and I have two raised beds. I've ordered a remote thermometer so I can monitor temps and open the ends when I need to. Temps this week are in the 40's with lows in the 30's so not too worried about heat yet. So far I've planted lettuce, arugula, various bok choi type greens, radishes, chard. So cool weather crops.

One question is when things start to heat up, do you pull the hoops or use perforated white shade fabric to keep the things like lettuce a bit cooler?

My tomatoes are actually doing great this year and I have about 100% germination on the seedlings. True leaves are just starting to emerged but not sure if I should leave them under the fluorescents or move them outside into the hoop in three weeks or so. Do I need to harden them off before putting them in the hoop?

Good looking pics! For the hoop house, I've been using fabric instead of plastic so I don't have to vent it on warm or sunny days. The agribon ag-70 works fine in cold and snowy temps. Not sure about the shading cloth. I might experiment with that myself this summer for lettuce, etc.

yes. you still need to harden off the tomatoes because they will be getting more sun, wind, and temperature changes. I don't put mine in full sun for more than a couple of hours at first. I use a place in the yard that gets shaded during the hottest part of the day. They all sit in a garden cart so I can wheel then into the garage depending on weather. I don't plan out on a sunny day. I try for either overcast or late in the day. I have a portable low tunnel hoops but cover in Agribon fabric so it can stay on 24/7. It takes 2 people to move it but I completely remove later in the season before plants get too big.