What to Do in the Garden This Week - May 21st

Published on May 21st 2020
purple allium flower bulbs
Now that the soil is warmer, you can get those cold-sensitive plants going. And it's not too late to plant a hanging basket or two or even more! With the whole summer ahead of us, now is the time to plant for colour!

Hanging baskets

  • It's not too late to plant up a hanging basket! If you haven't done it before then choose a basket with solid sides so that you only need to plant up the top.
  • When choosing plants for hanging baskets, aim for a central bushy upright plant - the "thriller". Then add 3 or 4 trailing plants - the "spillers". Finally, plant some bushy growing plants to cover the area in between the thriller and spillers. These of course are called "fillers"!
A hanging basket full of flowers
  • Use the best potting compost available to get a fuller and longer display.
  • Add resin-coated, slow-release fertiliser tablets (Osmocote, Grow-Sure, etc.) so that your plants are well fed for most of the summer.
  • Water regularly and nip off any flowers that have gone over. This encourages more flowers to be produced.

Greenhouse and poly-tunnels

  • Temperatures under cover can get very high so, besides plenty of ventilation, you'll need to put shade on the outside. Apply shade as spray-on shade paint or as netting to the roof of your greenhouse.
  • If your crops have suffered from red spider mite attacks in the past, damp the floor down regularly to raise air humidity. Don’t do it late in the day as this can encourage diseases. If you're going to use natural pest predators, introduce them as soon as you see the first pests.
Red spider mite damage on a cucumber leaf
  • Companion plants will encourage natural predators into your greenhouse and these will control pests. I find that pot marigolds or tagetes have worked very well for me. They work especially well on whitefly which can be extremely difficult to control in other ways.
  • Plant melons. They need it hot and humid so they are good for polytunnels. They also like soil with lots of organic matter in it and masses of water!
A melon fruit

Borders and cut flowers

  • Trim foliage off early flowering perennials to encourage fresh new leaves. Lungwort (Pulmonaria) responds particularly well and it gets rid of the powdery mildew that they often get.
  • Tie up sweet peas regularly and remove unwanted side shoots. If you have the time, do remove the tendrils too. Watch out for greenfly and check the small print carefully if you plan to use an insecticide as some will harm your sweet peas.
  • Hostas can be divided this month. Lift clumps and split them, making certain that each shoot has plenty of roots attached.
Hosta Gold Standard plants
  • Watch out for scarlet lily beetles. This pest can eat your lily leaves in a few days! Pick them off and squash them. However, the larvae are hard to see and I use Provado Ultimate Bug Killer. A non-chemical alternative that will protect your plants is Grazers Beetle Control, but you'll need regular sprays.
Lily beetles


  • Sow sweetcorn directly into well-cultivated soil outside. Sow in blocks, not rows, for good pollination. F1 “Sundance” is a particularly good cropper and has been awarded RHS Award of Garden Merit.
  • Sow courgettes, pumpkins, squash, marrows and ridge cucumbers one seed to a pot. All like lots of organic matter so plant them with lots of compost. These can be grown on top of the compost heap.
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