When to use a plastic bag for seeds?

I notice in some seed conversations, it is recommended to pop a plastic bag over the pot (creating a mini greenhouse effect? )

But, is it necessary with all seeds?

 

It seems to be the 'modern' thing to do Mum. They survived well enough before plastic was invented. I don't use plastic bags, well I didn't until last autumn. I've used them to hold some heat in a couple of pots that have clematis seeds, which have over wintered in my cold greenhouse. I usually rig up my own makeshift cloche in there but it wasn't necessary this winter.

 

I'm more confused over understanding which seeds may need it and which may not and how to tell between them

I have been saving pop and water bottles to make mini cloches, but, I'm not at the stage of needing those (yet).

 

Most seeds don't need it but it depends on when you start the seeds off and in what situation. 99% of my seeds are started off in an electric propagator at this time of year and as soon as they sprout are transferred to my spare room, but giving as much sunless light as possible. Late March I will transfer them to the greenhouse. The others, like clematis that are long to germinate (6 months to 3 years) or need a cold spell outside are left pretty much to their own devices in the greenhouse.

If you start them off in March to April in a greenhouse they will germinate in their own time and won't need covering. Quite often seed packets will indicate requirements. Some specialist plants however have particular needs.

It's really a case of trial and error Mum and taking/not taking the risk. If you'd like to list any seeds you are worried about then I/we can advise you.

 

Thank you, Sheal.

Basically, I bought a (reduced!) pot/compost/cosmos.

I potted up the first pot - even though the seeds didn't look very good - and the "advice" was: "put in a warm place". No advice about lighting levels and definately no suggestion of a plastic bag. So, it has been put in a warm spot indoors, with moderate light, is being kept moist but nothing else.

To be honest, I'm not expecting any kind of positive result, judging from the condition of the seed packet. This is purely an experiment to see *if* I have green fingers and can bring something seemingly moribund into something "Eureka!"

Then, seeing suggestions about plastic bags, made me wonder if perhaps the cosmos would benefit - even though that wasn't suggested.

Just to confirm:-
~ no greenhouse
~ no coldframe
~ most windowsils are north facing
(well, you don't buy a "learner" driver a Ferrari, do you )

 

Well you haven't got the easiest of situations to grow them in Mum, but if they come up and there's no reason why they shouldn't, that'll be power to your elbow!

You won't need plastic bags on them indoors, it's usually to give them some help if they are in a cold situation. As it happens I'm growing Cosmos for the first time this year and they're in a queue for the propagator at the moment.

 

Basically if it's not in my heated propagator I put 'em in a placcy bag. The only real benefit is that it stops the compost from drying out anything like as quickly.
Essential when using paper towel instead of compost.

 

Should have added that if using placcy bags it's even more essential to sterilise the soil properly.

 

Anything I start off indoors I either put in a plastic bag, or put a clear plastic propagator lid over the top. Any moisture that evaporates from the soil condenses (on the plastic bag, or lid) and runs back to the soil, so it doesn't need any watering until it germinates (although if it takes more than 3 months to germinate it may well need some watering over the following months).

In olden days the advice was to cover the seed tray with a sheet of glass, which had the same effect - the water vapour condensed on the glass and dripped back onto the seed tray (although I think a dome cover is better as the water runs down to the sides rather than directly onto the compost). Newspaper was used instead if no glass available, It reduced the evaporation but not as well as glass, and a plastic bag does a better job IMHO.

As soon as the seeds germinate open the vents on the pastic lids, or open the bag (to allow fresh air to circulate into the bag, this will reduce the humidity level, otherwise the seedlings may succumb to the high moisture vapour levels), and over the next few days open it wider and then remove it. Water from the bottom (if you water from the top it will encourage algae etc. to grow on the surface, and potentially Compost Fungus Gnats. Use tepid tap water, not rain water (too many bugs etc. in rain water, for seedlings with undeveloped immune systems to cope with).

Feel the weight of a pot / seed tray, put in a tray of tepid water for 5 minutes, lift it up and feel the weight - it should be much heavier? Experience will tell you what "weight feel" is on the dry side, and what "weight feel" is moist enough - and even what weight is "too wet"

Don't leave the bags / trays with plastic lids in direct sunlight such that the contents can cook! but make sure that the seedlings have plenty of light as soon as they germinate (keep a record of days-from-sow-to-germinate so next year you know when to start checking

 

very useful information