In your ornamental garden in May

General tasks

• Plant out dahlias after the last frost has passed.

• Turf new lawns.

• Take a regular walk around the garden and look out for potential signs of pest and disease More information about pests & diseases here

• Protect plants against slugs with various forms of barriers, traps and ferric phosphate pellets.

• Greenhouses and conservatories can heat up quickly in May – shade with greenhouse shading fleece or greenhouse shading spray. Open the doors and vents during the day.

• Mulch sweet pea plants with well-rotted manure or compost; this will conserve moisture and restrict weed growth.

• Stake herbaceous perennials to stop them flopping.
• Keep plants well watered during dry spells to avoid them becoming stressed and susceptible to pests. Save as much water as you can in water butts, and try to avoid having bare soil – make full use of mulches and ground cover plants.

Greenhouse

• Many of the seedlings you have raised can be planted out after the threat of the last frost has passed
• Be sure to harden off plants gradually before planting out or standing out.
• Make room for softwood cuttings of Penstemons and Pelargoniums, etc.
• Sow seeds of hardy biennials such as evening primrose and foxgloves and perennials such as Achillea, Alstroemeria and hardy geraniums.
• The temperatures should be high enough to use all types of biological controls in your glasshouse. Use the predatory mite Phytoselius for red spider mite, the tiny wasp, Aphidius for Aphids, the predatory mite Hypoaspis for control of sciarid fly, the parasitic wasp Encarsia for white fly and Cryptolaemus beetles for mealybugs. All require 18-20°C for a couple of hours a day, so make sure you can achieve these temperatures before you buy.

Biological controls are available from the Organic Gardening Catalogue.

Garden Organic members have online access to our factsheet:

• Red spider Mite
• Sciarid fly
• Aphids

Access to these factsheets requires members' password.
Find out more about Garden Organic membership.

Lawn Care

• Lawns will be growing vigorously now. Try leaving the box off the mower, and let the clippings fly. They will rot speedily in warm weather, returning valuable nitrogen to the growing grass. You'll save time and effort if you don't have to go back and forth emptying the mower box. In dry spells, don't cut the grass too short. A height of 3cm is ideal.

• If you are thinking of buying a new mower, consider a 'mulching' or 'recycling' mower. These machines chop up the mowings very finely, then spread them evenly over the lawn. No more clumps of mowings walked into the house, and much time saved not having to empty the grass box.

• Now is an ideal time to lay turf or re-seed a lawn. Prepare the areas by levelling, raking off all stones, tamping down to firm the soil and a final light rake to finish. Sow seeds or lay turf on this prepared soil, and then water.
• If you are laying turf then water until it is established, or water in particularly dry periods through the summer months.

Pond management

• Tadpoles and newtpoles should be maturing and looking to leave the pond soon, make sure there is a sloping side to your pond to ensure they can get out.
• Try growing watercress in your pond, it is a British native and prefers slow moving water but will also perform well in a pond. Simply buy some from the shops and place a couple of sprigs in a glass of water and it will produce roots in a few days. When it has rooted, plant out in the margins of your pond either in the water or the mud at the edge. This will give you a good crop throughout the summer months entirely for free. If we have another mild winter it can come through and grow again for you next year.
• There are many other edible aquatics that you can grow in your pond.
  Garden Organic members see our factsheet on Edible aquatic plants.

What to plant in May

Hakonechloa macra 'Aureola'

Milium effusum 'Aureum'

Deschampsia flexuosa 'Tatra Gold'

Miscanthus sinensis

May is the very best time for planting grasses and bamboos. These plants only grow new roots in late spring and early summer, so now and into June are the very best times for planting.

• There are many grasses to choose from, from small tufty clumps to tall wavy, elegant grasses.
• You can find grasses that are suitable for full shade or full sun and anywhere in between.

• For the shade try Hakonechloa macra 'Aureola' also Milium effusum 'Aureum', found by E.A. Bowles at Birmingham Botanic Gardens. This is a beautiful delicate golden grass worth a place in any garden.

  Deschampsia flexuosa 'Tatra Gold' is a small lime green, clump forming grass ideal for dry shade.

• Grasses for sunny spots are easy to find, if you have the room try a large Miscanthus, these are generally tall upright grasses with feathery flowers. Try Miscanthus sinensis or if you like your grasses tall and tropical looking try Miscanthus x giganteus.

Make new plants

Now is also the best time to dig and divide grasses and bamboos that have grown a little too big for their space.

• Gently lift a grass from the ground with a fork.
• Shake off any excess soil from the roots.
• Take a sharp knife and cut through to centre of the clump.
• Replant half immediately and water well.
• Divide the other half into as many pieces as you like and replant or pot up. Water well for the next few days until the grass is growing again.
• Sow seeds, either direct for hardy annuals or into modules for hardy perennials. This is the very best way to sow your perennials, as root damage from pricking out is non-existent. Plants establish quickly and are easy to pot on or plant directly out.

Pseudosasa japonica


Phyllostachys nigra


Phyllostachys aureosulcata f. aureocaulis


Phyllostachys vivax f. aureocaulis


Phyllostachys aureosulcata f. spectabilis

Easy Perennials

This section gives you our suggestions of perennial plants that will look good or are looking good now, are easy to grow and fairly free of pest and disease, making them a good bet for an organic garden. All plants marked with a * indicates an Award of Garden Merit (AGM) from the RHS.

This month we will look at bamboos. These are popular plants, easy to find and architecturally useful in the garden. They are fairly difficult to propagate and this is why bamboos can seem a little expensive. Once planted and settled in they grow easily and seem to be entirely free of pest and disease.

• We'll start with the plainest of bamboos, Pseudosasa japonica* it has plain, large green leaves and green stems. It will grow in sun, shade or part both. Pseudosasa japonica is easy to find and one of the cheaper bamboos to buy. It is vigorous but not too invasive and ideal if you have a couple of square metres to fill.
• One of the most popular and most expensive bamboos is Phyllostachys nigra*, or the black bamboo. It has jet-black canes and deep green leaves. It is easy in sun or shade, and usually does not grow much in the first year or so after planting. It then takes off and will produce several new canes each year and bulk up nicely. It is not really invasive and seems to behave itself quite well, forming a lovely stand of tall upright black canes and delicate foliage.
• The yellow or golden bamboos are also very popular and their vibrant yellow canes add colour and structure to any garden. Phyllostachys aureosulcata f. aureocaulis* is a tall dense bamboo with pure yellow canes and small leaves. Again it does not seem to be too invasive and will form a good-sized stand in 5 years or so.
• Very similar is Phyllostachys vivax f. aureocaulis*. It has thicker more knobbly stems than P. aureosulcata and is slightly more robust in its shape and form, and thus requires a bit more room that your average bamboo.
• The final golden bamboo to recommend to you is the very stunning and striped stemmed Phyllostachys aureosulcata f. spectabilis*. This is one of the most sought after bamboos and is very expensive to buy, but worth every penny. The canes are incredibly striped green and yellow and sometimes a bit of a zigzag between each node giving a truly exotic effect. It is fairly vigorous but seems to behave quite well and if it does out grow its space then friends would gladly accept offsets.

Pest watch

• 
  Greenfly symptoms

• Check your plants regularly
  Infestations can develop overnight at this time of year - don't let this happen. Small colonies of aphids can be wiped out easily between your fingers. Puckered leaves are a good indication that greenfly (or some other colour aphid) are feeding on the undersides. They won't kill the plant, but will spoil the appearance.

• Black Spot - Remove rose leaves with the classic blotching of black spot on sight. This material should be put into the dustbin.

• 
  Vine weevil adult

• Biological Control - The soil should be warm enough now to use nematode treatments against slugs and vine weevil larvae. Don't apply the nematodes in dry weather - they need moist soil particles to swim through. The vine weevil nematode is very effective in containers at this time of year.

• Start nightly patrols to hand-pick slugs, snails and vine weevil.
  
  
• 
  Ladybird eggs
  
  
  Hoverfly larvae
  
  
  Ladybird larvae
  
  
  Hoverflies on fennel

• Here come the predators

  Beneficial insects, such as ladybirds and hoverflies, will be seen as the month progresses and the weather warms up. Look for ladybird eggs on plants near aphid colonies. Female ladybirds can lay over 1000 eggs, in batches of 20 - 50. If you're lucky, you'll see adults, larvae and eggs on the same plant.

  Hoverfly larvae can be mistaken for pests. They are voracious aphid eaters, so will be found in the middle of an aphid cluster. Adults feed on nectar, then lay their eggs near aphids so that when larvae hatch, they have a food source.

  Check very carefully before either squashing or spraying. There will be beneficial creatures around now, munching happily on pests on the undersides of leaves. Insecticidal soap is just as damaging to them as it is to the pests.

  If you haven't sown any predator-attracting plant seeds yet, it's not too late. Seeds of Eschscholzia (Californian Poppy) and Helichrysum (Strawflower) can still be sown. All sorts of useful, predatory insects will feed from their nectar. Flowers of the Umbellifer family of plants, such as carrots and fennel, are particularly popular. Their flowers will be used as a feeding station by hoverflies and ladybirds.

  Garden Organic has produced a factsheet called Attracting Beneficial Insects (requires members' password) which lists the top ten insect friendly plants that every gardener should know about.

• Online access to our organic factsheets is password-restricted to Garden Organic members'.
  
  Find out about becoming a Garden Organic member here
  
  You can download the complete list of factsheets with order form (PDF document - 101Kb)

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