If they have been outside since they came up - then they are already acclimatized - no worries.

Hardening Off is about conditioning plants for the total experience of being outside in nature. Wind, full strength UV radiation and cold nights. Some plants only need week to get with the program others take a little more. You'll just have to watch them to see which ones are handling it better. Plants that have never been in full sunshine or wind will wilt, so you want to start out in the shade (bright shade) and keep them watered. What I've done in the past was to trim leafy branches from weed trees and stuck them into the ground around my freshly planted seedlings. The umbrella of shade would last about a day (different trees handle this better than others). Right now a lot of trees haven't leafed out yet - but privet, eleagnus and ligustrum work well. If they wilt completely I will prune out some new ones and replace them each day. Sounds like a lot of work but my yard has tons of weed tree saplings and I was born with pruners in my hand so it isn't that much trouble. Anyway with twice a day sprinkling I could keep most of the saplings providing some shade for the transplants. After a few days I let things slide a bit and by the end of the week I yanked out the sticks and let them sunbathe all they wanted. If you keep your babies in flats (highly recommneded until mid April), you could just keep them in a shady part of the yard at first and then move them out near where you intend to plant them. If the weatherman says its gonna get cold (32 or below) then I would either cover them all up with newspapers and mulch or old bedsheets and if you're desperate - new bedsheets. Or simply move them up beside the house wall or near a door or window (an amazing amount of warm air leaks out of windows and doors). If I notice one of the plants is handling things without problems then I go ahead and plant it. Some take a while to adapt, others don't.

Greenhouse plants are not hardened off, even without shade cloth they are not getting enough UV to be ready for full sun. Most of my selling area is out in the open, so I have to harden off my greenhouse grown plants before they get moved in to stock. I mostly grow ands sell cacti and succulents, with a few weird native and xeriscape rarities thrown in, so I am running a specialty line that may not be comparable to the average nursery. But any plant that is meant for outdoor use I try and have pre-hardened before it is sold. However there are times that I do bring plants straight out of the greenhouse and put in stock under a shade awning, when I do, any customer who buys one of those plants is given an info-card on how to harden off plants successfully. Small "shade cloth and bamboo pole kits" are available for those that want to get their plants in the ground right away, without waiting to harden off. Any shade-kits returned in good condition receive a store credit. By making sure that we talk to every customer and be proactive with them, I help them realize why they should keep buying from a small local grower-retailer, rather than picking up half dead "bargains" at one of the local big box stores. When they ask why my plants look so good compared to the" bargains" it is usually easy to make them understand what local grown, in soil designed for our climate (not Southern California where the big box stores import from) means.

I put my seedlings out into full sun in the yard three days after potting up, but... 1) There can me no wind, or it will beat them to death. After a couple of days in high light they toughen up and can take more and more wind. The wind, not light is the reason people start out slow, putting them out for only a few hours at a time the first few days. 2) Soil must be saturated with water. If it dries out and the leaves wilt they will sunburn. 3) I put the plants on grass/lawn, not concrete patio/driveway. Heat will build up on the concrete and fry/dry them, the lawn has high humidity. Not all of my seeds germinated befor I potted up the largest seedlings so some of the dirt attached to the roots had seeds in it, some of those those seeds have germinated in the pots out in the sun and the sprouts are doing fine. Temps have not gone above the mid 70s here, I might be more reluctant if the temps went higher than that. Shoplights with 2x4ft fluorescent tubes are usually only around $10 at many stores. You can use foil as a reflector around the plants to boost the light. hope this helps