Perennials in June

June 1, 2018

By: Ron Kushner

June is a good month to plant perennials but it can get very warm at the end of the month. If the weather is unusually hot (over 85 degrees F.) your plants experience more transplant shock and that could set them back a bit. Make sure they are watered, maybe (3) times a week until new growth appears. Then cut back watering to once a week, deeply. It is best to check daily for any wilting.

Make sure any plants started (or bought) in pots are planted in the ground now, if that is your intention. Try to have all plants in the ground by mid June. If you divide and transplant this month, cut the plants back and keep them well watered.

Keep up with your weeding. Place any supports and stakes now that may be required later in the summer. Japanese beetle adults will be out this month. Be vigilant and knock them off into a bowl of soapy water early in the morning. Cut back any flowers that bloomed in early spring that may be flopping or looking straggly and cut off dead or yellowing leaves.

Any plants that will bloom later in the summer or fall can be pinched at this time to create additional growth and to control excessive height. Joe-Pye weed is a classic example. By pinching, I mean simply removing the actual growing tips, including one set of leaves on each shoot. Using your fingers is usually the best way.

  • Mums (Chrysanthemum spp.): Pinch back or cut back early in the month. Pinch off any buds that are showing at this time. In the Philadelphia area and other zone 6-7 areas, pinching should end before July 1st. Mums need about a month to set new buds after pinching.
  • Roses: Cut a bouquet for inside the house. It will strengthen the plant and promote healthy growth. Deadhead ever-bloomers. If Japanese beetles can’t be controlled by hand picking, spray with neem oil twice a week.
  • Yarrow (Achilla millefolium): Deadhead young plants and they will bloom all summer. Cut plants back by 1/2 early in the month and you will get shorter, self-supporting plants. Caution: if you cut back after the buds form, you will not have any blooms until next season.
  • Hollyhocks (Alcea rosea): Cut off yellowing leaves and any damaged, diseased or eaten by insects.
  • Aster: Cutting back early in the month will fill them out and they will flower a bit later in the fall.
  • Delphinium: Make sure stakes are in place. Thin plants when they are 6″ tall so as not to crowd each other out. They are heavy feeders and can use a bit of fertilizer at this time. A slow release, granular, balanced fertilizer is best.
  • Coneflower (Echinacea spp): Prune early in the month only if you want to postpone blooming time until late in the fall.
  • Native sunflowers (Helianthus spp): You can cut back some this month so some will be shorter than others. Blooming will still happen all summer into fall. Heliopsis helianthoides can also be cut back but blooming will be delayed until later in the summer.
  • Candytuft (Iberis sempervirens): Blooming should be finished this month. Shear back the entire plant and keep watered through the rest of the summer.
  • Shasta Daisy (Leucanthemum X Superbum): Deadhead now and continually. Blooms will continue into the fall.
  • Peony: Deadhead immediately after blooming.
  • Indian cup plant (Silphium perfoliatum): Cutting back the stems now will make the plants stronger. They will still bloom all summer until the frost kills them or the finches eat all the seeds.
  • Goldenrod (Solidago spp): Cut back this month. Cutting back will reduce their height but it will create a bushier plant so you may not need a stake.

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