One of the main jobs in the flower garden during May is to harden off and plant out all the hardy and half-hardy bedding plants that you have been raising under glass. The actual time for planting will vary depending where you live but, as a basic guide, the second half of May is usually safe for the southern half of England whereas, in the north and Scotland, you will have to wait until June to avoid all danger of frost.
The way to harden off plants is to give them progressively cooler conditions until they become used to outside temperatures.
This is bestachieved by giving the plants more ventilation in the greenhouse or frames or, if they have been raised indoors, by putting them outside during the day and bringing them back in at night. Ideally this process should continue over a ten-day period.
Although nurseries, garden centres and DIY stores will have tender plants for sale this month, don't be tempted to plant out your purchased half-hardy subjects too early. If you buy plants too early for planting then keep them in a cold frame and harden them off as you would for home grown plants until it is safe to plant them outside.
Clear spring bedding such as wallflowers and recycle them on the compost heap. However tulips, hyacinths and other bulbs can be lifted, when the leaves have died down, dried off and saved for replanting in autumn.
Window boxes, tubs and hanging baskets can be planted up in May ready for placing them out in the garden. The aim should be to provide instant colour so the plants must be well in bud or actually flowering when they are planted. Use a light compost for hanging baskets e.g. not one containing loam such as John Innes.
This is the time to plant out these flowers. The site should be thoroughly dug and well enriched with bulky organic matter (well-rotted garden compost, etc). Just before planting, a dressing of general fertilizer, such as Growmore, should be given at the recommended rate. As both these plants need the support of a cane or stake, the latter should be pushed in the ground after the hole has been dug but before the plant has been settled in.
Hardy Annual that were sown in the borders last month will need thinning soon, after the seedlings are large enough to handle, to avoid any risk of overcrowding. Remove the weakest plants and retain the best and strongest plants. After thinning, give the seedlings a really good watering to settle them in and to help them recover from the upheaval.
Many annuals can be transplanted to tubs, window boxes or hanging baskets now.
Geranium (pelargonium) and fuchsia cuttings taken in spring can be moved into larger pots and hardened off in May, ready for planting out towards the end of the month.
Half-hardy annuals grown from seed or bought ready-grown in trays should also be hardened off. If you are using a cold frame, give the plants as much fresh air as possible. Raise or take the covers off during the day, and return them at night when the temperature drops.
Heliotrope and old-fashioned cannas are particularly frost-tender, so keep them under cover for an extra week or two.
Herbaceous plants that are going to grow taller than 30 cm (1 ft) high should be supported when they are still only a few centimetres tall. Ideally this can be done with twiggy pea sticks, where subsequent growth will quickly hide them. Plants that need supporting in this way include Michaelmas daisies, border phlox. Tall hardy annuals can also benefit from this method of support.
Taller plants such as delphiniums will need stems supported by canes. Three or four bamboo stakes per plant will give strong support as the young plants grow, take strong twine from cane to cane and criss-cross it through the centre.
There are many other useful plastic and metal devices on the market now that can be used for plant supports.
Continue sowing herbaceous plants outdoors. Also, many of next spring's bedding plants can be sown now if you have room in the garden. A dedicated area of the garden such as a nursery bed or in the vegetable plot is the most convenient place, where they can be sown in rows and transplanted later.
You can also sow hardy annuals during May in readiness for a late autumn display. Although the main sowing period for hardy annuals is March / April, those plants will finish flowering at the height of summer, so a later sowing now can extend your display well into autumn.
May until July is the time to start off biennials from seed ready for next spring's display. The sooner in the season you sow biennials the better the plants will become. It is best to provide good-sized, strong plants before the autumn cold sets in, as once the cold weather hits all the plants energy goes into flower production. Two exceptions to the rule are Brompton stock and pansies, which should not be sown during May as they can flower prematurely, providing a weak show next spring.
Because biennials take much longer to reach the flowering stage, compared to annuals, it is best to start them off in a nursery bed and move them next autumn to their final positions. Alternatively, if you don't have the space you can try an annual variety such as Sweet Williams, which can be sown in spring ready for summer flowering.
Never allow weeds to become established in spring, especially amongst annuals and bedding plants. They will soon overtake and ruin them unless you keep then in check by hand weeding or using a hoe.
May is the best time to lift, split up and replant any alpines that have finished flowering and are showing signs of becoming tired and worn out.
Any hardy perennials or alpines sown from seed early in spring can be planted out now, but they are unlikely to flower this year, so keep them in a nursery bed. Alpines that flowered last month can do with a trim, to tidy up their appearance. Cut back hard, as it really is a case of having to be cruel to be kind. They will put on rapid new growth and some will even reward you with a second, late summer crop of flowers. You can make more alpines now, by dividing them into several small plants. Use two kitchen forks, placed back-to-back, to gently tease the roots apart.
Daffodils, tulips and hyacinths may well be past their best at the end of May, but can still taking up valuable space in the garden. If the bulbs are in the flower beds and not in a naturalised area of the garden it is now time to move the bulbs is you want to get on with bedding out summer plants, or sowing another batch of hardy annuals.
If the bulb's leaves are still green, it means that the plant is producing and storing food for next year, which will in turn provide the energy for flower production next year. Therefore they should be replanted in an out of the way spot (but still sunny) until the goodness in the leaves and flower stem has found its way back into the bulb.
To temporary heel-in bulbs dig a V shaped trench, about 15cm (6 in) deep. First remove any spent flower heads, then lift the bulbs carefully, without damaging the fine white roots. Carefully place the bulbs in the bottom of the trench, with their leaves and stems pointed upwards. Replace the soil and firm it, taking care not to crush the stems. Apply slug pellets or slug repellent to prevent slug damage. Label the spot so that you will remember where they are once the foliage has shrivelled away and not forget to dig them up later.
Once the foliage has fully died down naturally they can be lifted and stored indoors until the autumn. Clean them carefully, rubbing off any soil still sticking to them, and store in a single layer in a dry, cool place.
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Herbaceous perennials in flower this month:
Aquilegia (granny's bonnet)
Dicentra (bleeding heart)
Doronicum (leopard's bane)
Polygonatum (solomon's seal)
Trollius (globe flower)
Bellis perennis (daisy)