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May Garden Tips

May brings an explosion of flowers with, rhododendron, iris, peonies, astilbe, viburnum, spiraea, mock orange, and flowering hawthorn putting on a fabulous show along with fragrant lilacs, brightly colored azaleas, rhododendron, and many other beautiful trees and shrubs. This is the month when Pittsburgh gardeners will be taking advantage of the longer days to complete a variety of yard and garden tasks that will pay off with an attractive home landscape over the coming summer and fall months. May can be a busy, but make time for enjoying the beauty of the spring flowers and the birds and butterflies that follow the warm temperatures north.


  • To find out if the soil is ready to be worked, scoop up a handful and squeeze. Does it ball up or fall apart in your hand? If water trickles out, or if it keeps its shape when squeezed again, then it's too wet to work. If it feels dry or crumbles, then get out the tiller or shovel and start breaking up the soil for planting. Your soil test will tell you how much fertilizer and lime you need to work into the soil although all gardens will benefit from addition of compost, and other organic matter. (Call Penn State Co-Op Extension to obtain a soil test kit.412-473-2540)
  • For maximum landscape interest in a small vertical space, try annual vines. They can disguise ugly walls and fences. When trellised, they can create shade and privacy while hiding undesirable views. Try these annuals: morning glory, moon vine, nasturtium vine and scarlet runner bean. Perennial vines/shrubs: clematis, climbing hydrangea, and wisteria.
  • Plan a landscaping project on paper first. Check the mature size of each plant and allow for future growth. If you need design help, give us a call. Members of our staff, who have Master Degrees in Landscape Design, are very willing to help. Home visits and free 30 minute in store consults are available.
  • Work to eliminate the weeds (roots and all), before they have a chance to go to seed, or you will be fighting them for rest of the year. The compost pile should be getting a lot of use these days, both in utilizing this prime garden resource, and adding fresh garden refuse to it. The compost pile should be kept damp. Frequent turning will turn your garden waste into rich compost.


  • Watering roses with soaker hoses or drip irrigation will reduce the spread of black spot disease. It is important to keep the foliage dry.
  • Mulch around newly planted trees and shrubs. This practice reduces weeds, controls fluctuations in soil temperature, retains moisture, prevents damage from lawn mowers and looks attractive. A three inch layer is sufficient.
  • Remove spent blooms from broad-leafed evergreen plants so that the plants energy can go to foliage growth and next year’s flowers, rather than seeds. This is also the perfect time to fertilize if it hasn’t been done earlier. Use a formulation that has an acid reaction such as Espoma Holly Tone. Espoma Holly Tone should be used on all acid loving plants such as, dogwood, holly, junipers, rhododendron, azaleas, and pieris japonica.
  • Monitor pines, especially scotch and mugo, for sawfly caterpillar activity on new shoots. If found they may be removed by hand or use Captain Jack’s Deadbug Brew.
  • It's still not too late to fertilize your trees and shrubs. Use Espoma Tree Tone on deciduous shrubs and trees. Be sure to water the fertilizer in thoroughly after it is applied.
  • Early flowering deciduous shrubs such as Forsythias, Weigela, and Spiraea should be pruned back when they have finished blooming. Cut back a third of the oldest canes to ground level, then cut back one third of the remaining branches by one third of their height.
  • Work lime in the soil around your Hydrangeas to produce pink flowers or Aluminum Sulphate for blue.
  • Remove any sucker growth from fruit trees as soon as they appear.
  • Keep a vigilante eye on the roses. Keep them sprayed for aphids and other pests and diseases such as black spot. Plant Knockout roses, and forget about black spot.
  • Lilacs should be pruned lightly after they finish blooming, removing some sucker growths and dead blooms. Feed lilacs in May with Espoma Tree Tone after they have finished blooming. If your soil has an acidic pH, work a little lime into the soil as well.
  • May is an excellent time to plant a shade tree or flowering tree in your yard. When planting, be sure to incorporate Black Forest Soil Conditioner into soil backfilled around the plant.
  • Newly planted trees and shrubs should be watered deeply every week until fall rains begin
  • Rose maintenance: Remove all leaf and plant debris, and discard them in the trash, as they might harbor insect eggs and fungi. All roses are heavy feeders so this is a good time to fertilize with Espoma Rose Tone scratched into the mulch area around the root surface. . An alternative would be Bonide Guard & Grow which is a combination of a systemic insecticide and fertilizer. Guard & Grow will help prevent insect infestation later in the summer, as it feeds your rose.
  • Most roses would also benefit from an application of a fast acting foliar spray fertilizer throughout the summer. Leggy stem growth and very few flower buds is a sign that the plant is lacking sunlight. Watering is also very important to roses; the spring rains are usually adequate to support lush new growth. As summer brings sunny hot weather, it is very important to water these plants, thoroughly soaking them at least once a week with a hand wand. Be sure to apply water to root area and not to foliage; water lying on foliage will cause fungi. If overhead watering is necessary, be sure to water early in the morning so foliage has a chance to dry out. Also keeping a thick layer of mulch on them will help to hold more moisture
  • Pruning of evergreens: The best time to prune evergreens is just after the new growth has flushed out. By cutting or pinching back the candle growth, the plant has an easy time recovering and setting new buds for next year. Some plants will flush new growth more than one time a growing season. Keeping this new growth cut back will create thicker foliage. Shearing is appropriate only when a hedge or formal appearance is required. Rejuvenation pruning is the complete removal of older stems at the base to allow new shoots to replace older stems that are removed. 


  • Petunias, begonias, coleus, impatiens and many other warm season plants will be damaged by freezing temperatures and are not safe to plant until after May 20th.  Cool-season flowers such as snapdragons, alyssum, and pansies may be planted in early May.
  • To grow annuals in containers on the patio, use a light weight soil mixture. Keep the plants well-watered, because the soil dries out fast. Apply a water soluble fertilizer according to package directions every week to two weeks.
  • Resist the urge to cut back the foliage on your spring-blooming flowers after blooms fade. Tulips, daffodils, and other spring bulbs need the foliage to make large bubs for next year's blossoms. Wait until the foliage turns yellow which is normally expected around 15 June.
  • Begin planting warm-season annuals and summer bulbs such as caladiums, dahlias, cannas, after 15 May. Understand though that frost is still possible for a couple of weeks. Be prepared to cover tender plants with a cloth if the temperatures is expected to drop below 36F.
  • Begin fertilizing container plantings. Continue at regular intervals using Shultz All Purpose Plant Food, or Neptune Fish and Seaweed fertilizer.  This is critical for container plantings and hanging baskets, which require weekly fertilization.
  • Planting annuals: The last average spring frost date in the Pittsburgh area around May 15th. Memorial Day is a safe time to plant all annuals. Even though annuals can sometimes be a lot of maintenance and need to be planted every year, there are no other plants that give you so much color all summer long. All annuals are very heavy feeders so preparing the bed is very important. Start by turning over the soil and mixing either compost or any other organic material into the soil at about a 50/50 rate. Add a slow release fertilizer by scratching it into the top layer right before planting. The best time to plant these small transplants is on a cloudy day or late in the day so new transplants will not be stressed from the sun. It is also very important to water these small transplants at planting and also follow up daily for the first week to get them well established.
  • Pinch back mums to promote bushy compact growth and more flowers


  • Plant ground covers under shade trees that do not allow enough sunlight to grow grass. Vinca minor, English ivy, ajuga, and pachysandra, are ground cover plants that grow well in shady spots.
  • Remove all the dead wood and stalks from last year's perennial plants, taking care not to break off new growth.
  • Lightly sidedress perennials with an Espoma Garden Tone fertilizer. Avoid spilling the fertilizer on the plant, and use care not to damage the shallow roots when you cultivate it into the soil.
  • Setting the stakes next to your taller flowers early in the season, will help to support the plant against winds as well as making it easier to 'train'. Install plant supports for peonies, delphiniums and other tall, floppy plants before the plants get too tall.  The growing plants will hide the supports and plants won’t be damaged during a sudden thunderstorm. 


  • Planting starts with preparation of the soil, including a soil test to determine the soil's pH, or acid level, and nutrient needs. Most vegetables prefer a soil pH of 6 to 7, which is key to "unlocking" nutrients in the soil.
  • Turn the soil to a depth of eight to ten inches, thoroughly incorporating any leftover plant residues or manure you've added. Break up any clumps. Remove stones and sticks, then rake smooth from the soil surface to create a good seed bed.
  • A good rule of thumb, if you're a beginning gardener, is to follow the planting directions found on the back of the seed packet or in a gardening guide. For transplants, make sure they're hardened off properly before setting them out in the garden. To harden off, gradually expose your tender seedlings to colder temperatures and direct sun, by using a cold frame or by bringing your plants outside for several hours a day. Remember to cover them at night or bring them back inside. Don't let them freeze! Reducing water and fertilizer also helps harden of plants.
  • While just about all vegetables can be planted in the garden this month, timing is critical. Tender transplants like tomatoes, eggplants, and peppers may not survive if set out too early, and they usually don't grow much when it's cool anyway, so in most locations in the Pittsburgh area it's best to wait until the mid month to plant these in the garden. Plant tomatoes in a different spot each year to reduce fungus disease problems.
  • Pumpkins, squash, melons, and (most) many other vegetables and flowers also are sensitive to frost. Wait until the end of the month to plant these. We normally plant our pumpkins between 1 and 10 June.
  • Onions and peas--if you didn't plant them in April--as well as lettuce, carrots, root crops (carrots, turnips, beets), and hardy herbs can be planted through May. So can Cole crops--broccoli, cabbage, cauliflower, and Brussels sprouts-though be sure to harden these seedlings off before moving them into the garden.
  • Frost can injure all of these crops, so if one is predicted, cover plants with burlap, straw, or upside-down containers, bath towels or sheets. Plastic sheets will not provide adequate protection. Commercially available products such as hot caps or floating row covers promote faster growth by warming the growing environment during the day but usually offer only a few degrees of frost protection at night.
  • Grass clippings can be used as mulch in flower beds and vegetable gardens if allowed to dry well before use. Never use clippings from a lawn that has been treated with a herbicide.
  • Four or five layers of newspaper will serve as effective mulch in the garden. Cover the paper with mulch, dried grass clippings or straw.
  • Thin plantings of carrots, lettuce and beets to avoid overcrowding.
  • Control caterpillars on broccoli and cabbage plants by handpicking or use biological sprays such as Captain Jack’s Deadbug Brew.
  • Set out peppers, sweet potatoes and eggplants after soils have warmed which is normally the third week of May.
  • Make new sowings of vegetables seeds to insure continual future production.


  • Time to plant apple, pear, peach and all other fruit trees. Many fruit trees require two different varieties to pollinate properly. Be aware of this and ask one of our nursery professionals to guide you in selecting varieties.
  • Fruit trees need to be sprayed on a regular schedule, although you do not spray your fruit trees when the blossoms are wide open. Bonide Citrus & Fruit Orchard Spray is the recommended natural product. To obtain more detailed recommendations, contact our Tree & Shrub Manager.  
  • Now is the time to plant strawberries, raspberries, blueberries and blackberries. Mulch blueberries with pine needles or sawdust.


  • Lawns maintained at the correct height are less likely to have disease and weed infestation. Kentucky bluegrass and tall fescue should be mowed at two or three inches in height. Mow frequently, removing no more than one third of the blade at each cutting. Setting your mower for a higher cut during the spring months will help the grass to grow in fuller and help choke out the weeds.
  • May is a good month to repair your lawn. Fill in the bare spots by slightly loosening surface of the soil and sow a good quality lawn seed over the area evenly. Tamp the seed, fertilize, add lime, cover with straw, and then gently water. Keep the patch moist until the grass is growing nicely.
  • Watering is very important for newly seeded lawn areas; new flowers, trees, shrubs, containers, and hanging baskets.  Even established planting and lawns may need watering during May if we go 7- 10 days without a good shower.
  • To eliminate wild onion, dandelion and other weeds that are currently growing in your lawn, use Bonide Weed Beater.  This product will not affect grass but will kill all broad leaf plants. Use it with care.
  • Stop grub damage with an application of Milky Spore. Excessive mole damage may be an indication of a large grub population.


  • Once the fear of frost is over, you can move houseplants outside to the deck or patio and enjoy them outdoors for the summer. It is best to gradually introduce them to more direct sunlight to prevent the leaves from being burned.
  • As the growth rate of your house plants increases with the warmer and brighter season, adjust your feeding schedule to provide additional food.  Feed houseplants with a good quality indoor plant food such as Shultz All Purpose Plant Food, or Neptune Fish and Seaweed which is a natural fertilizer. Keep that in mind though, that the overuse of fertilizers can cause root and foliage burn, as well as the death of the plant.
  • Check to see if your house plants are rootbound. Water them thoroughly and carefully remove them from their pots. If the roots have compacted around the outside of the rootball, it is time to repot. Carefully examine your houseplants for pests and problems. It is much easier to fight an insect infestation or disease in its early stages than to wait.
  • Mist your plants regularly. This adds to the humidity, keeps the leaves cleaner and healthier, and helps to prevent spider mites.
  • Check the leaves of your houseplants for insects or any other problems. Bonide Systemic Houseplant Insect Control will provide protection for 8 weeks. Organic solutions include Hot Pepper Wax, and Bonide Bon-Nee. If needed, bring in a representative plant sample in a sealed plastic bag and one of our plant specialists will seek to diagnose the problem and recommend the appropriate control.
  • Divide indoor plants , if needed, when new growth starts in spring


  • Scale crawlers become active this month and come out from under their protective waxy active coating. Infested pines and euonymus should be treated at this time with Bonide All Seasons Horticultural Spray Oil.  A non-organic alternative that may be more effective would be a systemic insecticide such as Bonide Annual Tree & Shrub.
  • Slugs will hide during the daytime beneath a board placed over damp ground. Check each morning and destroy any slugs that have gathered on the underside of the board. An alternative would be to use a granular product called Sluggo. Take steps to control them now, before they have a chance to reproduce and devastate your garden.
  • Lace bug: Lace bugs feed on the under side of the leaf surface with piercing, sucking mouthparts; the main plants affected are Azalea, Rhododendron, Andromeda, and Cotoneaster. The damage appears on the upper side of the leaf as white spotting. The insect itself and its black excrement on the under side of the leaf is proof of this occurrence. Often, this group of insects will attack the plants because the plants are stressed and planted in the wrong area. These plants prefer shady areas, so when they get planted in full sun, they get stressed. At this time of the year, this insect is in its nymph stage and an application of horticultural oil sprayed on the under side of the leaf will control this insect. These young nymphs take one month to complete a generation, and as many as six generations a year. The nymphs can also be controlled by using a systemic drench of Bonide Annual Tree & Shrub. It provides protection for a full year. The only drawback is that it takes a couple of weeks to be fully absorbed by the plant; only then will insects be controlled.  


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