There is some confusion in the gardening world as to what half hardy annuals actually are. So to make it clear… just remember that hardy annuals are twice as hardy as half hardy ones, which in turn are half as hardy again. OK?
Or… half hardy annuals are plants which grow, flower. set seed and die in one year and won’t withstand the winter.
The other difference between them and hardy annuals is that tend to be more glamorous, a little more exotic. They have a deeper relationship with the sun and this perhaps makes them somewhat racy.
When Can I Sow Half Hardy Annuals?
In my neck of the woods the old boys say the last frosts finish by the 15th May. It is imperative that half hardy annuals are not sown outside before this day in our area.
For some plants this would leave too short a growing season and it is for this reason that the seeds should be started off undercover, in a greenhouse or similar.
Many start their seeds off in February. I think this is too early, seeds sown later soon catch up and often the early sown ones have problems, get leggy and/or are a general pain in the bum.
I tend to have a mass sowing session around the beginning of April. This gives the seedlings a full six weeks to get up to my ‘last frost date’ of the 15th May by which stage they will be big enough to handle the outside world and all its horrors.
However they will need hardening off. This requires you to get them acclimatized to the cooler weather beyond the safe confines of the greenhouse, windowsill or cold-frame. This can be done by leaving them outside during the day and into early evening for ten days or so before bringing them back inside for the night.
How do I sow my half hardy annual seeds?
The secret to this game (please don’t broadcast this) is to use 3inch square pots. “Why so?” I hear you sing in unison. Well my friends… a three inch pot ends up being just the right size for a six week old seedling, any smaller a pot and our chum the seedling will outgrow it and any bigger a pot will just take up more space, more compost and more of your hard earned pennies.
“But Benjamin…why in Berti’s name do they have to be square?!” Because it’s harder for the bl***dy snails to hide amongst. Line all your square pots, preferably in a tray with their sides close against each other….there is saftey in numbers.
It is worth noting at this point that Zinnia’s hate root disturbance AND they hate the cold… fussy little tykes. Therefore I sow mine a little later so they end up still being fairly small by my last frost day (15th May). I also harden off the other more macho plants first and leave the Zinnia’s till everything else is planted out.
Some folk like to sow in seed trays and then prick out and pot on. Yawn yawn… why make more work for yourselves. I’ve found germination rates are very high with single seeds in my three inch pots, if germination hasn’t happened after the others are sprouting nicely, I simply pop in another seed. Simples.
I use a standard peat free potting compost. We NEVER use products with peat in… if Kew gardens can avoid peat then so can we.
I cover the trays with a compost bag or thick bin bag to exclude the light until the very first green shoots can be seen. Don’t remove the bag too late as they will all get leggy and it will result in rubbish plants… your neighbours will laugh at you over their avocado and prawn starters and the whole summer will be ruined.
When can I plant out my half hardy seedlings?
Check the weather. The wonderful BBC have a weather site which will tell you if the temperature is likely to drop below freezing. It will be of no surprise to you to know, if it does… don’t put ’em out. I had a disaster last year, if you’re feeling like having a snigger at my misfortune then please click on FROST DISASTER!
Aftercare of half hardy seedlings
Much like my hardy annuals, I plant out at 25cm apart and water in really well. Let them get their roots right down amongst the action and become established where they will find the ground water.
I use pea netting as support for my plants, some folk don’t. I suggest if your plot is remotely unsheltered then you would be better off with some support. If you want to use your blooms for cut flowers then it is a must as this helps keep the fellas straight.
Can’t I just wait until the frosts have finished and plant my half hardy seeds in the ground?
Most half hardy annuals perform much better with a push start in the greenhouse. Some however will do just fine when sown after the frosts in the ground where they are to grow. Cosmos seem to be fine with this approach as do Zinnia’s but they will flower a couple of weeks later.
Most importantly, enjoy yourself… plant some too early or too late, see what happens, or perhaps turn them into funny hats and go to the pub for a port and lemon?
Don’t forget… you can always buy flowers off me if yours don’t work out. ;)