To save money on plants, I often buy smaller ones in April, but keep them in the greenhouse until the last frosts are over around May 15th. B&Q is usually best value and range, but I also use Woodies, Dairygold, Hanleys and Kiernan's, as well as The Pavilion.
You will need
- A big space to spread stuff out
- A tarpaulin so everything doesn't end up covered in compost
- A bowl of warm water, washing up liquid and scrubbing brush
- Pieces of broken crock
- Farmyard manure
- Slug pellets
- Osmocote long-acting fertiliser
- Water crystals
- Watering can/hose
- Bucket of water
This is to avoid diseases and pests from last year and make them look nicer on the outside. When you've washed them, lay them on the tarpaulin the way that the windows on your house are arranged so that you don't forget which colours will be next to which, or where you might have a shadier window that needs different plants.
Prepare the bottoms
You need to put crocks over the drainage holes. Wait, did I forget to mention, check that your window boxes actually have drainage holes? Some cheaper ones do not. You can get someone good at DIY to drill holes if needed, otherwise your plants will drown. If you don't use curved bits of old broken pots and plates over the holes, roots will block them and... your plants will drown.
For the bottom of the pots, I usually use a mixture of organic farmyard manure and topsoil. If you only use compost, when it's dry your plants will not get enough water, and also the boxes may not have enough weight to be stable on your window sill in a gale.
I find that if you want your window boxes to last, you need to add water crystals to ensure the plants stay moist, and also osmocote slow-release fertiliser. Slug pellets to dress the tops of the soil after planting are also important.
There is at trade-off between environmental damage caused by effective chemical slug pellets, and loss of plants to slugs if you go a more "organic" but less effective route. Each to their own on that debate! I add the fertiliser and water crystals to the topsoil/manure layer before planting.
Just get all the plants out and lay them roughly in position, thinking about colour, which windows get most sun, and which side of the box is the "front" where you will put the trailing plants. Move them around until you are "least dissatisfied". It will never be perfect. I use surfinias, fuchsias, osteospermums, trailing lobelia, verbenas and I'm fond of good old reliable yellow bidens too.
Get your fingers dirty!
Some plants these days are raised in "tea bags", small individual paper mesh containers of compost. These need to be removed carefully before planting. I like to then soak every plant's roots briefly in a bucket of clean water before planting. Gently loosen the plant's roots before placing it into the box.
The next bit is messy. Shovel in the potting compost, firming it around the roots and making sure there are no "holes". I usually push the trailing plants like lobelia in last of all, directly into the potting compost. At this point your boxes probably look awful and you think you did it wrong. Don't worry. They just need watering.
Once you spray off the boxes ,all will be well. I usually use a watering can from the top followed by spraying the sides of the boxes with a hose. Don't use too forceful a jet. Test the hose first - I find a gentle spray with a circumference of about 6 inches works well. After watering, sprinkle on the slug pellets or other slug and snail prevention.
Now they are ready to go to their final location. If it is a damp summer, I use old bits of floor tile to raise up the window boxes just a small amount to assist in drainage.