May Gardening Tips

May gardening tips

<< See April Gardening Tips 

Each month, there’s so much that could be done in the garden that it’s hard to know what must be done. So I’ve broken the May gardening tasks into three categories. If you only have time for a few things, focus on the ‘Must Do’ tasks. Add in the ‘Should Do’ and ‘Nice to Do’ tasks as time allows.

Plants at their best in May

Alchemilla mollis (Lady’s Mantle)
Allium (ornamental onions)
Calendula (pot marigold)
Clematis montana
Dianthus (pinks)
Digitalis (foxglove)
Geranium (perennial varieties)
Iris (Siberian and Japanese iris)
Oriental poppy

Must Do

There’s really not much that absolutely must be done this month; it all depends on what you want out of your garden.

  • Mow the lawn regularly, cutting off no more than 1/3 at a time.
  • Weed – You’ll see this gardening chore appearing on the list every month! To make it easier, check out our review of the Best Weeding Tools.
  • Watch out for insect pests – May is when many insects start to emerge in large numbers. If you notice an infestation, take action immediately. One particular pest to watch for is the bright red lily leaf beetle which, if left unchecked, can quickly decimate your lilies. Simply crush the beetles or knock them into a can of soapy water.

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Should Do

  • Spread mulch – Some people recommend mulching in early April before weeds emerge. I prefer to wait until May, when the soil has warmed up and plants are growing well, before spreading about 2 inches of compost or leaf mold on all exposed soil. Yes, you’ll have to weed first, but I think plants develop better when the soil is allowed to warm, rather than staying cool when covered with mulch in April.
  • Deadhead rhododendrons and lilacs – To encourage blooms for next year, remove old flowers from rhododendrons and lilacs.
  • Plant gladioli, dahlias, and summer bulbs – For information on how to do it right, see our Resource article on Planting and Growing Bulbs.
  • Plant containers and window boxes – Plant containers of annual flowers in mid-May, fertilize well, water, and watch them grow.
  • Prune early blooming shrubs – Immediately after blooming, prune shrubs like rhododendrons and azaleas to shape them and reduce size. If you’re looking for a good online resource about pruning, visit the Complete Pruning Guide.
  • Plant and sow vegetables and annuals – Most vegetables and annuals can be planted this month. Don’t wait too long to sow vegetables with a longer time to maturity, such as sweet corn or beans, or they may not fully mature before the fall frosts arrive.  You may want to hold off on planting tomatoes and peppers until the end of the month or early June when the soil has warmed up.
  • Pinch back mums – Pinching encourages stocky, branching growth with more flowers.
  • Spray roses – Many roses have problems with blackspot and require regular treatment with horticultural oil and/or fungicide. An organic version I like is GreenCure fungicide – read the review here.

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Nice to Do

  • Fertilize azaleas and rhododendrons – After the petals drop, apply a specialty fertilizer like Holly-Tone from Espoma.
  • Give your houseplants a vacation – After Memorial Day, bring your houseplants outside for the summer. Be careful not to place them in full sun to prevent burning.

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Note: The May gardening tasks described here are for gardeners in zone 6. Tasks may be done earlier, or later, if you live in warmer or colder areas. I’ll be adding more information over time showing tasks in other areas of the country. If you have suggestions, please share them in the comments below!