May Zone 4

What to do in the garden in May in Zone 4

Average last frost date is May 15 in Zone 4

  • Apply dormant oil  An application of dormant oil to shrubs and trees early in the month will kill most insect eggs.  But this MUST be done while plants are dormant and air temperature will be above freezing for at least 24 hours.
  • Fertilize bulbs  Spring blooming bulbs should be fertilized in April or early May.
  • Plant bare root trees  Bare root trees, shrubs and roses should be planted as soon as the soil is thawed and dried, usually in early April but sometimes even in March.  Sometimes the weather hasn’t cooperated until early May, but now time is running out.
  • Transplant  Move shrubs and trees after the soil thaws out and dries up if it hasn’t been done already, before new growth begins, and the weather is cool.  Transplant shock will be minimized.
  • Plant container grown shrubs and trees  By end of the month, container grown trees, evergreens and shrubs can be planted.
  • Spray fruit trees  Apply dormant spray to fruit trees before the buds swell, if you didn’t do in April.
  • Tune up the mower  It may be too late to beat the rush, take it in early in the month.
  • Mow  Get on a regular mowing schedule.  You should not be removing more than 1/3 the length when you mow.
  • Deadhead  Snip or shear off only the dead flower heads of spring blooming bulbs if you haven’t done it yet.  Do not cut back the foliage until it has withered and yellowed.
  • Clean up  Finish any perennial cleanup and removal of winter mulch.
  • Divide perennials  As soon as the ground has thawed and dried in April or early May, you can divide and move perennials.  Do not divide the very early spring bloomers such as bleeding heart until after blooming or in fall.
  • Clean up perennial grass  Cut back the dead top growth of perennials and perennial grasses if you didn’t get it done last month.  Leave about 3 or 4 inches of stems that will help keep hungry rabbits out of the new growth and keep you from stepping on them.
  • Prep the beds  When the soil has thawed and dried and begins to warm up, usually around middle April but sometimes not until May, you can remove winter mulch.  Work compost or leaf mold into the top layer of the soil.
  • Plant bulbs  Plant summer and fall flowering bulbs as soon as the soil has thawed and dried.  Hopefully you were able to do this in April, but you can still plant them early in May.
  • Plant perennial seeds  Seed can be sown for frost tolerant perennials as soon as the soil has thawed, dried, and begun to warm up in April or May.  Some seeds however will not germinate until the soil is quite warm, so check directions on your seed pack.
  • Prune roses  Early in May (usually in late April) before growth begins, prune dead, broken and wayward branches from hybrid tea, grandiflora and floribunda roses.  Cut back to about 6 inches tall  When the forsythia blooms, just ahead of dandelions blooming and lilac leaves appearing, it is time to prune roses.  Apply a slow release granular fertilizer.
  • Plant cool season annuals  Plant cool season annuals like pansies as soon as the ground has thawed and dried.  You may have been able to do this in April.
  • Stake plants  Stake tall plants that need support and protection from wind before they get to tall to confine.
  • Plant container grown trees, shrubs and roses
  • Fertilize the lawn  Depending on weather patterns, this could be April or May.  After the first mowing, apply fertilizer.  Pre-emergent for crabgrass should be included if necessary.  Crabgrass seed generally germinates after the soil temperature has reached 50 degrees, and requires about 5 consecutive days of 50 degree soil temperature.  When you see the first dandelion bloom, it is time to apply crabgrass control.
  • Apply broadleaf weed control  Apply broadleaf weed control in May.  Although you can purchase fertilizer with broad leaf weed control, conditions should be ideal when you apply (read the package) for best results.  Mixing up some weed-b-gone in a tank sprayer is more effective.  It also allows you to spot spray individual weeds are target small areas.  Clover and creeping charlie require several applications.  With a pump sprayer you can target those every few to several days.
  • Aerate the lawn  Every few years the lawn should be aerated.  De-thatching should wait until late summer or early fall.
  • Harden off seedlings  Seedlings started earlier in the year can be set out early in the month on warm sunny days to harden them off.  The process is long, setting them out each day for longer periods of time until they are strong enough to be out all day and over night.  You may be able to start this process in late April.
  • Plant hardy annuals  Often by the first of May you can plant the hardy annuals such as geraniums and even petunias.  A light frost usually does them no harm but a freeze will.  Don’t rush to plant, if it is not warm enough not much will happen anyway.  But window box soil warms up quickly and stays warm next to the house, so your hardy annuals will get an early start establishing roots.  And containers or hanging baskets are easily moved into the garage if a freeze or hard frost is expected.
  • Transplant seedlings  Transplant any annual, perennial or vegetable seedlings into the garden beds after the last frost date.
  • Apply Aluminum Sulphate  The blue Hydrangeas should be treated with Aluminum Sulphate to keep them blue.
  • Visit garden centers  See what’s new this year before you start buying and planting.


  • Plant annuals  By last frost date you can plant all your annuals.  Depending on the year, a cold May can bring late light frosts, which can harm the most tender annuals such as impatiens.
  • Mulch  Apply a 2-4” layer of mulch on your planting beds to inhibit weeds and retain soil moisture.
  • Remove trunk protection  Remove any winter protection wraps from your tree trunks to avoid harboring insects.
  • Check containers and baskets  Container gardens, hanging baskets and window boxes dry out quickly and need frequent watering.  They will also need to be fertilized regularly.
  • Heat up the compost  If the compost isn’t ready yet but is not heating up, add green plant material such as grass clippings or fruit and vegetable waste, or add a shovel full of manure.
  • Check for insects  Keep an eye on your plants for insects that may be causing damage.  Do not indiscriminately spray insects, as many are beneficial.  Even those that cause visual damage may not necessarily be harming the plant.  Check with your local garden center or extension service to properly identify and treat for insects.  Routinely spraying them off with a garden hose or applying insecticidal soap is often the best course of action.  Or use a water bottle with a small holed tip on it that will produce a strong stream.  Aphids is a common problem.  Although they generally do not do permanent damage, they can inhibit growth and weaken the plant.
  • Fertilize flowering shrubs  Apply a liquid or slow release granular fertilizer to spring blooming shrubs after the flowering is complete.
  • Take down the coldframe  By the end of May you can put away the coldframe.
  • Drink water and wear sunscreen!
calendarSharon Dwyer