The Garden House Tasks for May
Posted:8 May 2025
Please bear in mind these tasks are not a comprehensive look at all garden aspects, but relate directly to our monthly work in the Garden House garden, inspiring and teaching the volunteers, students and Friday group gardeners who get stuck in and support the development of this unique Brighton garden.
By mid May we should be able to safely plant out half-hardy annuals – making sure they are properly hardened off (keeping close weather watch, late frosts can happen!).
Start to sow biennials towards the end of the month – Digitalis, Lunaria, Hesperis, Dianthus, (sweet William) and Wallflowers – these are very expensive to buy in the garden centre so it really is worth sowing some now. Keep them outside and plant out in October – they can remain in large plugs until then.
It’s also not too late to sow fast-growing half hardy annuals such as Cosmos and Zinnias to extend the season.
Bring out your Pelargoniums – feed with seaweed fertiliser every couple of weeks for a good flower display.
Plant out Dahlias and Chrysanthemums. At GH we are going to grow Chrysanthemums in pots and try Dahlias under a chicken wire cage until they are big enough to fight off the slugs. We have been spraying our Hostas with garlic spray, so far the results are good, and will try this on Dahlias too.
Chelsea Flower Show this month marks the time for the “Chelsea Chop” – cut back by a third perennials such as:
- Asters (Symphyotrichum)
- Echinacea.
- Solidago (golden rod)
- Perennial sunflowers like Helianthus ‘Lemon Queen’
- Achillea.
- Hylotelephiums
- Penstemon
Around this time we remove tatty-looking Allium leaves, leaving the flowering stalk.
Happily at GH we have a new greenhouse being installed this month. The plan is to use it for vegetables only – salads in boxes, gutters and enamel bowls – that way we can just cut them when we need them – like micro greens but a bit bigger!
We are also growing radishes and lettuces as well as strawberries and lemon grass, and trialing basil in hanging baskets and other tender herbs. Plus we are aiming to make this glasshouse plastic free.
At the end of the month it should be safe to plant out your tomatoes, here we are growing a variety of bush and cordon ones, some indoor and some outdoor – read our Friday Group Blog – on our website for more information about different varieties
May means weeding! Bindweed is rampant and also cleavers and other annual weeds – try to have a trug to hand as you walk round the garden for gathering the weeds, and don’t dispose into your compost bin!
Cut back Pulmonarias to encourage new growth and attractive foliage.
Divide Primroses after flowering, also Auriculas and move them to a shady position (Auriculas don’t like getting sunburned).
Divide Hostas now as they are coming into growth.
Remove pond weed with a net as water lilies start to spread their leaves over the pond.
Don’t be afraid to cut back trailers in alpine troughs and rockeries such as Alyssum, Aubrietia and Helianthemums when the have finished flowering to stop them becoming leggy.
Take softwood cuttings of tender perennials such as Salvias and other herbs, Argyranthemums, and Pelargoniums.
Feed bulbs with a liquid fertiliser to encourage good flowers for next year, this may also stop bulbs from coming up blind.
Pull out forget-me- nots and shake the seeds out once they have flowered.
This is such a wonderful time in the garden – enjoy it!