Garden Tasks for Mid- to late Summer (July - August)

Preparations

• Prepare the soil for anew lawn or border.
• Plant early-spring-flowering bulbs - like snowdrops, crocuses, and daffodils - in late summer.
• Take semiripe cuttings off shrubs.

Pruning

• Deadhead flowering plants to promote new blooms.
• Cut back herbaceous plants after flowering for a second burst of foliage and flowers.
• Prune rambling roses as soon as they have flowered.
• Prune established climbers and deciduous shrubs that flower on the previous year's growth.
• Trim evergreen and conifer hedges in late summer.

Other Tasks

• In dry spells, continue to water new plants and those still establishing themselves, as well as new lawns. Water in the early morning and evening rather than in the heat of the day.
• Water and fertilize hanging baskets and containers regularly.
• Fertilize and mulch roses.
• Continue to weed regularly.
• Mow lawns regularly - perhaps as often as once a week or more - but set the blades higher during dry, hot spells.

This list was found in Learn to Garden: A Practical Introduction to Gardening.  I did modify the list a little.   

Garden Tasks for Early Summer (June)

Preparations

• Feed flowering shrubs and roses with a rose fertilizer to promote flowering.
• Weed carefully to avoid damage to nearby plants.
• Buy bedding plants and plant out after the last frost.

Planting

• Plant hanging baskets and summer pots outside when all risk of frost is past.
• Plant out half-hardy and tender bedding plants after the last frost.

Pruning

• Prune late spring-flowering shrubs and climbers after they have bloomed.
• Trim vigorously growing hedges, like privet.

Other Tasks

• Remove suckers from roses.
• Tie in climbers regularly to their supports.
• Divide congested clumps of primulas and irises after they have flowered.
• Continue to take softwood cuttings.
• Mow lawns regularly and lower the blades.
• Water new plants in dry spells.

This list was found in Learn to Garden: A Practical Introduction to Gardening.  I did modify the list a little.   

Garden Tasks for Mid- to late Spring (April - May)

Preparations

• Apply a general-purpose fertilizer around established shrubs and roses.
• Mulch the soil after a rain or watering.
• Stake herbaceous perennials as growth emerges.

Sowing Seeds

• Sow seeds of half-hardy and tender annuals inside, or buy plug plants and grow on in a frost-free place.
• Harden off bedding plants sown earlier in spring.
• Sow hardy annuals in situ.
• In late spring, thin hardy annuals sown inside.

Planting

• Plant evergreens, and move any that are in the wrong place.

Pruning

• Prune early-flowering shrubs and climbers after they have bloomed.
• Cut back ornamental grasses as they start to grow.

Other Tasks

• Make new shrubs and climbers by layering.
• Start taking softwood cuttings from new shoots.
• Sod or sow new lawns, and repair worn-out lawns.
• Apply a spring fertilizer, and a weedkiller, to lawns.
• Cut the grass with the blades quite high.
• Keep weeding.

This list was found in Learn to Garden: A Practical Introduction to Gardening.  I did modify the list a little.   

Garden Tasks for Early Spring (February - March)

Spring officially beings March 20th; however we have had a very mild winter here in TN.  Just the other day it was 65°F.  I have several bulbs beginning to surface.  Anyhow, here's a look at tasks to be done in early spring.

Preparations

• Start weeding.
• Prepare soil for planting by forking it over and digging in organic matter.

Sowing Seeds

• Sow seeds of half-hardy and tender bedding plants indoors in a frost-free place.

Planting

• Plant summer-flowering bulbs, hardy perennials, and ornamental grasses.
• Plant bare-rooted shrubs and garden trees.

Pruning

• Prune bush roses.
• Prune deciduous  shrubs and climbers that flower later in the summer, such as buddleja, jasmine, lavateras, and certain varieties of clematis.  
• Prune hardy evergreen shrubs and trees.

Other Tasks

• Lift and divide oversized clumps of perennials; discard dead or unproductive sections in the middle of the clump.
• Prepare soil for planting in fair weather.
• Rake the lawn, if needed.
• Repot or top-dress shrubs, climbers, and alpines growing in pots and containers.

This list was found in Learn to Garden: A Practical Introduction to Gardening.  I did modify the list a little.   

Garden Tasks for Winter (December - January)

Yes, there are many things to do to keep your garden beautiful, even in the winter.  

Preparations

• Dig organic matter into clay soils.
• Protect slightly tender perennials with straw or compost.
• Brush heavy snow off limbs to avoid snapping branches.

Planting

• Plant deciduous trees, shrubs, and hedges on dry, frost-free days, unless the soil is very wet.

Pruning

• Prune trees (except for plums and cherries) and wisteria.
• In late winter, coppice shrubs for special effects like color stems or foliage.
• In windy gardens, prune back the topgrowth of tall shrubs to prevent windrock.

Other Tasks

• Check stored dahlias, cannas, and gladioli, and remove any that show signs of rotting.
• Wrap up pots with bubble wrap and cover vulnerable plants with fleece if frost is forecast.
• Continue to take hardwood cuttings of deciduous shrubs.
• Take root cuttings of perennials.
• Continue to rake leaves off the lawn.
• Keep off lawns and soil when sodden or frozen.
• Check and repair garden tools.
• Begin making plans for next year's garden.

This list was found in Learn to Garden: A Practical Introduction to Gardening.  I did modify the list a little.   

Garden Tasks for Fall (September - November)

Yes, it's getting colder and the daylight is fading, but there is still much to do.

Preparations 

Remove dead and dying stems and foliage from perennials.  Be sure to to leave a few to protect the roots over winter.

Planting

• Plant hardy perennials and hardy evergreen shrubs in early fall.
• Plant deciduous shrubs and trees late in season when leaves have fallen.
• Remove and compost summer annuals.
• Plant spring-flowering biennials and bulbs.

Other Chores

• Transplant deciduous shrubs in late fall.
• Lift and divide hardy perennials.
• Lift and store, or insulate, tender perennials and shrubs before first frost.
• Lift and store dahlias and cannas after the first frosts.
• Take hardwood cuttings of deciduous shrubs.

This list was found in Learn to Garden: A Practical Introduction to Gardening.  I did modify the list a little.