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Alan's Monthly Garden Calendar - May Ornamental Gardens

Published on 5th May 2019

May is THE big month for planting and sowing! Tender bedding plants, dahlias, begonias, pond plants all get the green light for planting out this month. There's more seed sowing, a bit of pruning and a few tips on how to keep on top of those weeds. Have a great time in the garden this month and don't forget to celebrate Garden Day with me on Sunday, May 12th!

Pots, baskets and borders

  • It's time to plant up your hanging baskets and containers. Think about colour combinations and try something new! Some plants do better in shade than in full sun. Good plants for shade are Lobelia, fuchsias and begonias. There are many suitable plants for sun, but trailing petunias, ivy leaf geraniums, Helichrysum, Verbena and Bacopa are reliable.
  • Harden-off annual bedding plants ready to plant out soon. Put them in a shady spot, keep well watered but bring them back inside if frost is likely. Keep a close eye on them, and they will be toughened up ready to plant after about a week.
  • Be ready to cover tender plants with horticultural fleece if frosts are forecast. Check the forecast every day this month.

  • Stake border perennial varieties that are tall growing and those that tend to flop over. Either use pea sticks -prunings from trees and shrubs are good- or use purpose made support systems.

  • Cut Aubrieta and Alyssum back hard when it has finished flowering and give it a feed. They will produce new growth soon and remain more compact if this is done every year.

  • Plant out Canna, Cosmos (chocolate plant), dahlias and begonias. These can be damaged by frost if planted too soon.

Trees, shrubs, roses, and other hardy plants

  • Ensure all recently planted trees, shrubs, roses, etc. are thoroughly and regularly watered.

  • Prune early flowering shrubs back now so that they have time to create new growth and flower buds for next year. Large Forsythia can be cut back hard; flowering currants trimmed a little and Camellias trimmed to reshape them. Spiraea arguta, Chaenomeles, winter jasmine, Kerria, Choisya and Viburnum tinus will benefit from careful pruning and still have plenty of time to set flower buds for next year. Try to avoid the bun shape that looks neat but unnatural.

  • Large, over-grown early flowering clematis such as Clematis montana, alpina, macropetala and winter flowering species can be pruned surprisingly hard now. They rapidly bounce back!

  • Watch out for Viburnum beetle and spray with an insecticide as soon as seen. Try to find a spray that is harmless to natural predators and bees.

Soils, mulching and weed control

  • Hoe off weeds regularly, especially during dry spells. A Dutch hoe is perfect for this.

  • Control weeds on paths and drives with Weedol Pathclear or another contact and residual action weedkiller. If applied now you should have weed control until autumn. Avoid seepage onto surrounding plants and borders.

  • Bindweed shoots are beginning to emerge from the soil. Treat these shoots with Resolva 24h or Roundup before they climb up your plants and begin to choke them! Roundup Gel works well and is safer for your plants. Avoid contact with any plant that you don't want to kill.


  • Treat lawn weeds with a good lawn weed killer such as Weedol Lawn Weedkiller. May is the best time to do it because fast-growing weeds are easier to kill. However, do not put the cut grass on your compost heap until the fourth cut after applying weedkillers. By this time the weedkiller residue will have gone.

  • Mow lawns at least weekly. A little and often gets the best looking lawn!

The Indoor Garden

  • Some houseplants like Cymbidium orchids, stick yucca, false castor oil [Aralia], Aspidistra, ivies, ferns and begonias can be moved outside this month for the summer months. Put them in a shady spot until they get used to the brighter light outside and make sure they get watered regularly. There could still be frost this month so be ready to cover them or move them back inside.

  • Just like us, lemon and orange plants love being outside in the summer months! Once acclimatised, they will relish the sunshine and often put on new growth!

Ponds and bog plants

  • Plant aquatic plants this month. They should establish very quickly. Good ones that I recommend are arum lily, water forget me not, water buttercup, marsh marigold, Kaempferi iris and dwarf waterlilies.

  • If you already have a water lily and it's grown too big, you can divide it now.

  • Keep the water clean by regularly cleaning water filters. If the water goes green - called a bloom - put a little barley straw into a net and submerge it in the water. This should restore the balance and clean the water.

  • Oxygenating plants keep the water clean so plant more. Avoid those that are invasive and those that are banned. More info here (

Alan Down
Tintinhull, GB
Retired nurseryman. Candide contributor and garden writer. Blogger. Lecturer. Consultant.

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