Smithsonian Gardens’ Horticulturist Alex Dencker will share suggestions on perennials that make excellent additions to gardens AND help support the habitat ecosystem.

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Q & A

Q–How do you define “native”? 50 years? 100 years? More?  

A–What is a native plant? That is an important question.  Native plants are plants that were here before Europeans settled the land. These plants have formed symbiotic relationships with native wildlife over thousands of years, and therefore offer the most sustainable habitat. A plant is thought to be native if it has occurred naturally in a particular region, ecosystem, or habitat without human introduction. Geographic considerations are also important.  For example, Fothergilla is native to southern Appalachia, even western Maryland, but not where I am in central Maryland.  Yet I still call it native.  

So, my short answer to your great question – plants that where here before 1492 and east of the Mississippi.  

 Q–What native plants are deer-resistant? Do soap shavings in sachets work at discouraging deer?  

A–There are many home-manufactured deer repellents.  I’ve even heard of people putting human hair and soap into bundles. Based on my experience those things will work for a couple of days the first and maybe the second time the deer visits but after a while the deer is not very discouraged or impressed by those efforts. In my experience what has worked best long-term are deer-resistant plants in the buttercup family; plants with some kind of rough or bristly foliage typically in the Aster family; plants in the mint family; plants with silvery foliage or fuzzy foliage; and anything with thorns, although with roses for instance, even if deer leave the plants alone, they will eat the flowers.  

There are lots of deer repellents you can buy. Many are based on rotten eggs, garlic, or even some strange mixture containing blood plasma or a combination of all three.  Some have rosemary or cinnamon in them. Some smell really bad and they all taste bad to deer. They seem to work pretty well and last a long time–you might even get through the whole season with only having to apply the repellent twice.  

Many of the plants I spoke about in my presentation will get nibbled when they are small and just emerging from the ground in spring.  Spraying deer repellant on them then will help so the deer will not go after them.  As these plants grow, they develop many of the unpalatable characteristics I spoke of earlier.   

Most land grant colleges, extension services, and master gardener programs publish “Deer Resistant Plant” lists.   

Q–How do I keep squirrels from destroying my garden? 

A–There are numerous animal repellents you can buy that contain blood plasma. These are effective because they trigger a flight response in the animal out of fear of there being a predator in the area.   

Q–The leaves on my black-eyed Susan turned black this summer. What happened? 

A–A common problem of blackeyed Susan is angular leaf spot caused by a bacterium. The disease causes brown or black angular spots on the leaves which can expand to blacken the whole leaf. Infection begins on the lower leaves and moves up the plant. Angular leaf spot of rudbeckia is most common on the cultivar ‘Goldsturm‘.they can look unsightly by early fall. 

Try another black-eyed Susan species.   

Q–IRudbeckia subtomentosa an aggressive spreader?  

A–I have never seen it become aggressive.  The clump does get bigger and is somewhat muscular so it will take up some space quickly.  After 3 to 4 years I would dig the clump up and divide it either in March or April just as it is emerging, or you could do it in the early fall.    

Q–I have straight species Joe Pye. Is there any way to keep it shorter? Or from flopping over? 

A–Cut it back in early June, before Father’s Daywhen the plants are about 3 feet tall.  This should not affect the flowering time. 

Q—New York Aster, many say that’s good for rain garden.  Does that mean it needs a lot of water 

A–Many rain garden plants can go from normal garden conditions all the way to living perfectly well and happy planted in a place that is inundated with water.  That’s not the same thing as a swamp or a bog condition, however.  Inundation means that it gets lots of water, let’s say in late afternoon or dinner time and by the next morning the water is gone though the soil still might be slightly damp or wet.  

To specifically answer your question, aster is very happy in welldrained garden soil.   

Q– My purple sunflowers grow to 5 feet tall.  Should I cut them back in September after they finish blooming? 

A–If they stay upright and still look good leave them.  I assume you want to harvest the seed before you cut.   

Q–What time of year should you divide perennials?  

A–Spring is good because it is cool and moist. There is less stress on plant and the moist soil helps keep the root ball intact. Because the plant is just emerging there is also less damage to the leaves.  September is also good because it is cooler, again less stress on plantWater the garden before “lifting” and dividing the plants to help keep the root ball intact. If dividing perennials in the fall, please time the activity so the roots will have time to acclimate –about four weeks –  before the ground freezes.  

Q–Is nepeta what we call catmint?   

Yes, Nepeta is catmint.  One species, Nepeta cataria, is called catnip and is particularly attractive to catsThe ornamental catmints are different species, or hybrids.  I’ve heard from other gardeners, though, that when they cut back their ornamental catmints that if there are cats around, they will come over to investigate. 

Q-What is your take on Amsonia?  

A–Amsonia hubrichtii is a great plant with a thousand uses—I can’t say enough good things about it. Amsonia tabernaemontana var. salicifolia is not as dramatic as A. hubrichtii but still very nice. Amsonia ‘Blue Ice’ is a great cultivar.  

QAre any of these plants poisonous to children or pets?  

A– Good resources to determine if a plant is poisonous:  

Q–What native grasses and sedges do you recommend to attract birds? 

A–I think this is a solid list:   

  • big bluestem – Andropogon gerardii 
  • Switchgrass – Panicum virgatum  
  • Prairie Dropseed – Sporobolus heterolepis  
  • Indiangrass – Sorghastrum nutans  
  • Canada Wild Rye – Elymus canadensis  
  • Sideoats Grama – Bouteloua curtipendula  
  • Virginia Wild Rye – Elymus virginicus  
  • Big Bluestem – Andropogon gerardii  
  • Little Bluestem – Schizachyrium scoparium 
  • Eastern Star Sedge – Carex radiata 

Q–What type of mulch is best for natives?  

A–My favorite mulch for just about any garden situation is fine pine or pine bark mulch. I don’t like to use shredded hardwood. Fine pine stays very fluffy, stays and looks like mulch a lot longer.  If you put it down in the spring it will still probably be there next spring, whereas shredded mulch often decays by mid-summer.   

Fine pine is also great as a soil amendment when you are planting. 

Q–Is the Minnie Pearl Phlox cold hardy? May I plant it in fall and keep it covered till spring? 

A–Minnie Pearl is very cold hardy, probably down to Zone 5. No special covering or preparation is needed.    

Q– I live in dry California.  What do you consider a rain garden?  

A Consult state, county or city extension offices, master gardener programsor land grant colleges–many offer programs that give advice on installing rain gardens and some even offer cash incentives.  It is a great thing!!   

Q–Planting distance of new landscapes: would you plant to accommodate the mature size of the perennial even though it might take 3-4 years for the plant to reach its described mature size?  

AYes! Check sources you trust-local garden center, fellow gardeners, internet, etc.about mature size and plant accordingly.  Or plant closer and plan to move them earlier.    

Q–I’m planting milkweeds (A. tuberosa, etc.) to provide caterpillar food. What other native plants do you recommend for caterpillar food?  

A–Great questionYou would be surprised that there is not as much research on this as one might think.  A great place to go and learn a ton is the work of Dr. Doug Tallamy from the University of Delaware. He’s written two terrific books on the subject and you can find his lectures on YouTube.   

Insects in the garden are a great thing and they are fascinating.  Here are some butterflies and their host plants:   

  • Monarch: Any plant of the Asclepias genus, including Red Milkweed, Common Milkweed, Showy Milkweed, Sullivant’s Milkweed, Butterfly weed 
  • American Lady, Painted Lady: Ironweed 
  • Fritillaries: Various violets, Bird’s Foot Violet  
  • Tiger Swallowtails: Trees such as Ash, Birch, Chokecherry, Basswood, Poplar, Willow  
  • Black Swallowtail: Members of the parsley family (Apiaceae) including Golden Alexanders 
  • Pearl Crescent, Tawny Crescent: Smooth Aster, Frost Aster and other asters 
  • Baltimore Checkerspot: White Turtlehead 
  • Columbine Duskywing: Columbine 
  • Common Buckeye: Wild Petunia, Verbena stricta 
  • Mottled Duskywing, Spring Azure, Summer Azure: New Jersey Tea 


Q–What if you have a small, mostly shade garden? 

A–There are many websites out there with information on native plants for shade gardens.  These examples are just off the top of my head, all for shade, good for wildlife, and smallish.   

Columbine – Aquilegia canadensis 
Fire Pink – Silene virginica 
Native ferns – numerous 
Goat’s Beard – Aruncus dioicus 
Jack- in-the-Pulpit – Arisaema triphyllum 
Canadian Wild Ginger – Asarum canadense 
Yellow Trout Lily – Erythronium americanum 
Early Meadow Rue – Thalictrum dioicum 

Bleeding Heart – Lamprocapnos eximia 

Carex spp. 


Q–A lot of plants in the native garden seem to have purple or blue flowers.  What would be a good choice for plants with flowers that are orange, yellow or white to create a contrast? 

A–There are many websites to look at for information on native plants.  These natives with yellow, red, orange, white flower are just off the top of my head.  Many are my favorites. 

  • Canada Anemone – Anemone canadensis 
  • Columbine – Aquilegia canadensis 
  • Heath Aster – Aster ericoides 
  • Nodding Pink Onion – Allium cernuum 
  • swamp Milkweed – Asclepias incarnata 
  • White Aster– Aster ptarmicoides 
  • White False Indigo – Baptisia alba 
  • White Turtlehead – Chelone glabra 
  • White Woodland – Aster divaricatus 
  • Wild Quinine – Parthenium integrifolium 
  • Cardinal flower – Lobelia cardinalis 
  • Woodland sunflower – Helianthus divaricatus 
  • Purple Prairie Clover – Dalea purpurea 
  • Purple coneflower – Echinacea spp. 
  1. Mountain Mint – Pycnanthemum muticum 

Q–I have several Liatris spicata. Some have either gone dormant or died, or are still green.  This is the first year I have had them, so I don’t know if the ones that turned black are dead or just dormant.  Any thoughts? 

A–They are a pretty tough plant, though having too much soil or mulch on top will kill them.  The hard-bulblike thing that the green growth is coming out of is supposed to be just below the surface.  This is supposed to remain firm to hard.  If it is soft and mushy it is dead.   

Q– If my bee balm has powdery mildew, can I still save the seeds? 

A–Yes, powdery mildew should have no effect on the seeds or their future propagation.   

Q–Did you go over the carrot family plants? 

A–No, I did not, though I mentioned them at the start of the presentation because they are great at attracting pollinators and beneficial insects. I need to do another talk! A great native in the Carrot family (Apiaceae) is Angelica atropurpurea. 

Q–Can you spell the names of the shade plants? 

A–I think the shade plants I mentioned at the end were 1) Woodland Phlox, Phlox divaricata, 2) Creeping woodland phlox – Phlox stolonifera, 3) Golden Alexanders – Zizia aurea. 

 Q–After fall turns to winter, should you cut all perennials to the ground? Or are there some you should leave, or cut down only partially?  

A–Don’t cut the plants you think offer winter interestleave them as long as possible.  It would be a good compromise to leave things till say after Thanksgiving.  The date is up to you.  Then cut these herbaceous plants down to 1 to 1.5 feet.  At that height all you have are stems sticking out of the ground.  Many insects will use the stems of overwintering perennials as their winter hometheir spot for “hibernation.”  Cut them to the ground in mid-March.  Certainly, I understand and appreciate the desire to put things to bed at the end of fall and tidy things up before winter. However, for a healthy habitat, leaf litter is actually quite important as many beneficial insects and pollinators live there.