Weather and the Gardener
Although the increasing warmth of the sun and the rising soil and air temperatures promote growth in the garden in April, everything can be brought to a halt by one night of frost. Spring frosts are always a major threat when the wind comes from a northerly direction, when the sky clears and the wind dies away at dusk, especially when the soil surface is dry.
The most efficient defence against spring frost in the garden is some form of electrical soil heating, but a glass or plastic cover can provide some limited protection againsts all but the more severe frosts.
April should see the last of the spring frosts in most gardens , but the dangerous years are those when both March and April have been very dry months, in which case there is a serious risk of a frost in May.
Trees and Shrubs
Those in flower in the month of April include some forms of amelanchier, magnolia, malus (crabs), prunus (cherries, eaches, apricots, etc.), berberris (barberies), camellias, chaenomeles (Japanese quince), corylopsis, cytisus (broom), daphne, erica (heaths and heathers), forsythia (golden bell bush), Kerria japonica (Jew's mallow), mahonia, Osmanthus delavayi, pieris, rhododendrons (including azaleas), ribes (flowering currants), spirea, viburnam, and cleatis species.
Pruning
Complete pruning of plants that have flowered earlier in the year and those which flower in summer on new wood. remove straggly branches of evergreens such as lavandula (lavender), euonymus (spindle tree), and Magnolia grandiflora.
Planting
April is a good month to plant, or transplant, broad leaved evergreens. Plant pot-grown climbers such as wisteria, lonicera (honeysuckle), vitis (ornamental vines), and pot shrubs such as ceanothus and jasminum (jasmine). Water in newly set out plants, and continue regular watering during dry weather.
Roses
Apply a proprietary rose fertilizer at the recommended rate to established roses and lightly hoe it into the soil. All roses benefit from applications of liquid fertilizers at intervals from spring to late summer, provided the soil is damp. If dry, apply plain water a day or two beforehand. Remove weeds at the same time as feeding.
Flowers
Many plants can be divided and rejuvenated in April. Split up large clumps, that are becoming overgrown and flowering poorly, of the following; scabious, knifophia (red hot poker), helianthus (sunflowers) and helenium.
Continue staking leggy plants that are making strong growth. Use canes and string, or twiggy pea sticks.
Watch out for developing colonies of greenfly on soft shoot tips. If seen, spray with systemic insecticide in the late afternoon when bees and other pollinating insects have gone.
Alpines
Continue planting containerized plants. Work a mixture of equal parts moist peat and loamy garden soil among the crowns of cushion saxifrages and other spreading, shallow-rooted plants on light soil, to conserve moisture in summer and prevent leaves browning at the edges in dry weather.
Bulbs
Plant summer flowring tigridias, galtonias, acidantheras, outdoor freesias, Crocosmia masonorum and nerines. Set the bulbs and corms to a depth of three times their maximum width.
Leave faded and shrivelling leaves of spring flowering bulbs to die back completely before removing them from the plant. If this job is done too early, while the leaves are green or even yellow, muchof the food within the leaf will not have passed down into the bulb to strengthen it for flowering well the following year.