Plastic bags for seedlings
Lucy3
Posts: 92
Help.............I plan to sow my last summer perennials this week and all the advice says if you haven't got a propogator put the trays in sealed plastic bags..I can only find opaque blue sandwich/freezer bags that are large enough to hold some of my seed trays.....do they have to be clear or are are the blue ones ok? if I need clear ones where do you get them from that are large enough to hold the seed trays and also large enough to be able to tie them?....thank you 
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Hello Lucy3
I use the opaque blue freezer bags for seeds and cuttings, and find they work very well (I get mine from Asda...) They are just the right size for a half seed tray, and I think their "opaqueness" helps protect the seed / cutting from too much direct light. I have a row of them on my kitchen windowsill as we speak.
I cover mine in supermarket carrier bags and they seem to do fine.I take them out as soon as I see the shoots coming through.
Also take a shifty look in the tall, wheeled isle bins that often stand near the fresh dairy product isles while the staff are stacking and clearing the shelves. You may spy excellent quality clear bags being thrown away, some of substantial sises that you can put 2 or 3 full size trays in. Also, look out for clear plastic wedge shaped lids that go over 1 dozen packs of some yoghert type products. There is one in particular that is absilutely the same size as a seed trey, and it has a ready made "frosting" effect to diffuse the light a little. I'd tell you what brand it was but I have never actualy asked what it came off. There are also ideal multi-pot stands to be had, to lift 6 or 12 plant pots at a time (just make a mental note of the diamiter of your favourite plant pot bottoms and you are laughing). The staff are usualy only too delighted to ferret out a particularly usefull looking item if you explain it is for gardening purposes. If any bag, lid or multi-carton you see dosn't look pristine when you get it home wash it with a very mild detergent and rince it well. The inside of bags can be done in the same way. If I come across a good, tough transparent one I will wash it carfully and re-use it several times. Sometimes I use detergent but if they are getting greenish or they smell I use a little bit (a puff at most) of anti-bacterial surface cleaner spreyed into the bag, rub it arround a bit and then rince out the bag thoroughly. I've never had any problems with mould after doing this. Finally, experiment carfully with freezer bags, which do have the massive advantage of being re-sealable. Most of them are perfectly Microwave prooff. So if you put a little water in the bottom and sit the bag, open and upright in the microwave it will steralise itself in a very short time. Check that they don't melt - some say microwave safe on the lable just to help you out. Also, goes without saying really, carfull lifing the hot, steamy bag out. A rince, followed by microwave will provide a recycled sterile bag if you don't want to have to keep buying new ones.
Lastly, if you can find a book or website that gives a general and reasonably comprehensive guide for different plants that need germination in light, or those that need shade, or darkness to germinate please let me know because a chart or lists would be very useful. I could put up in my potting shed for quick reference. Maybe Gardeners World Magazine could publish one???
Cling film is OK as long as you don't have pets or children. If you have cats or dogs and they manage to tread on your trays the cling film will be ripped. Children like to pierce the smooth shiny surface with an inquisitive finger, making a satisfying POP (and who can blame them). Also, you need to make sure that you leave an adequate gap between the soil surface and the cover for a little air. Both Clingfilm and tight plastic bags can cause some kinds of surface sown seeds to be pulled up by static building up in the plastic. Children (again quite understandably) like to rub the smooth, squeaky surface, and see little bits start to fly up and stick to the plastic! If you can adequately restrain or exclude children and pets I'm sure cling film is fine.
Thank you so much to all of you...................being a relatively new gardener I thought I would try and do it 'by the book' - when ever I have seen pictures of gardeners putting trays/pots etc in plastic bags they have always been clear ones but I know better.................I have a large roll of opaque blue freezer bags and can get on with my potting today in the confidence that my seeds will germinate...............I am also going to the supermarket tomorrow for the weekly shop..dare I say ASDA............and wiill be keeping a furtive eye out for all the surplus packaging products you have mentioned....................I am also new on here and this was one of my first posts and am astounded at all the help I have received.thank you so much...although you know this is going to now lead to morre questions for you alll being as you are all so knowledgeable gardeners...........Hope to tallk to you al again soon, wish me luck with the freezer bags............
Lakeland do really nice large bags - size 8, I think. I use them to keep my homemade bread in, but they fit well over seed trays
Probably too expensive to buy them just for seeds, but pinching a couple doesn't seem so extravagant 
I've also used cling film over small pots.
I have used super market bags for years and cling film over small pots. Always worked well for me provided you tuck them in to be air tight.
Lots of the plastic trays that desserts and ready meals come in make great seed trays and covers - I use the brown trays for seeds, and clear containers for things like tiramisu are often exactly the right size for a half seed tray cover. I pierce the 'base' containers with a thin screwdriver heated over a gas flame in order to make drainage holes without the plastic splitting. I use blue bags for covers, too!