This summer enhance your recipes with a few sprigs or leaves from an herb you’ve grown yourself. Herbs are easy to grow whether in a garden or in pots on a balcony, by experienced or first-time gardeners. Discover how to grow 10 herbs to flavor your summer from Smithsonian Gardens’ Horticulturist Erin Clark. 

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Q&A

Q–Why does parsley bolt? What does bolting mean?
Parsley is a biennial, meaning it focuses on leaves or vegetative production one year (or growing season) and on reproduction the next. If planted in the fall, it will often bolt by May in D.C.  This means it will send up stems with flowers that go to seed.  This signals the end of its life cycle and it will often start to yellow after this.  If you are able to start parsley in early spring and keep it happy in the summer months, harvesting leaves during this first season will encourage new growth.  You can also plant it in the fall as a winter or cool season crop. 

Q–What is an annual, perennial and biennial?  
An annual is a plant that in nature completes its life cycle of germinating, growing and flowering, and producing seed in one year.  You can say it bolts and then perishes at the end of the season.  

There are some plants that are more long-lived in their native tropical regions, but are considered annual in temperate zones because they perish in frost. Basil is a plant that completes its life cycle by flowering at the end of summer and then the leaves will experience damage or turn black when exposed to the first cold temperatures of the fall.

Perennials are plants that will come back year after year for a few or many years. They have a living crown at soil level and a root system that survives over temperate zone winters or they may be semi-woody, as is the case with lavender and sage and hardy rosemary and keep their stems above ground level over the winter.

A biennial plant focuses on growing leaves one year and flowers that go to seed (bolting) the next year to complete its life-cycle.  Since many biennials reseed in place, a patch could perpetuate itself for many seasons. 

Q—Is there a trick to growing rosemary in D.C.?
The key to growing rosemary in the mid-Atlantic region is to find a hardy variety using the USDA hardiness zone map. D.C. is in Zone 7, so you need a rosemary that is hardy to at least Zone 7 (surviving 0 degrees F lows in winter) or colder—Zone 6 or so.  Since it hails from the Mediterranean, rosemary does not experience cold temperatures in its native region.  By cultivating varieties and trialing them over many winters, American growers have found a few hardy varieties.
Hardy varieties include: 

  • ‘Arp’ (named for Arp, Texas) hardy to -10 degrees F, upright, can grow 4 feet tall and wide. This is widely available and the most hardy. 
  •  ‘Madalene Hill’—sometimes sold as ‘Madeline’ or ‘Hill’s Hardy’ or Hill Hardy or Hardy Hill, hardy to 0 degrees F, 36-40” tall 
  •  ‘Athens Blue Spires’ or ‘Blue Spires’ for Athens, Georgia, 3 feet fall, 2 feet wide, hardy to -5 to -10 degrees F 
  •  ‘Alcalde’, developed in New Mexico, 30-36” tall, 24-26” wide, hardy to below 0 degrees F 

Grow your rosemary in a sunny spot with good air circulation next to a wall that is east, west, or south-facing. This will help protect against wind and reflect heat. Brick is especially good at reflecting heat.  

The site also needs good drainage, as constantly wet soil in winter will weaken rosemary.  Mulch with shredded hardwood or gravel to protect the roots from extreme cold and heat and prevent soil from splashing back onto the leaves during watering and rain.  Water deeply when the soil starts to dry out.

To trim to use or to shape for a hedge, prune off stems only after any danger of frost (mid–May to mid-October here in D.C.).  Sage and lavender are also semi-woody sub-shrubs that shouldn’t be pruned in frost-prone months. 

Q—The most durable lavender?
The most durable lavender cultivar depends on your individual garden conditions and preferences. In the mid-Atlantic, you need to watch out for wet winters and summer humidity. We grow lavandin cultivars like Lavandula x intermedia ‘Provence’ and ‘Phenomenal.’ I have had some first-year summer loss and winter loss with ‘Provence’ with some plants failing and others thriving. However, English lavender is generally preferred for culinary uses. The culinary varieties that do well for us are English lavenders like Lavandula angustifolia ‘Munstead’ and ‘Hidcote.’ There are many new and old varieties to explore. 

Q–Can basil grow in partial shade?
Basil does best in a well-ventilated garden space that receives full sun (6+ hours a day). It can survive in a warm, sunny window or possibly in partial sun (3-6 hours) but partial shade (2-4 hours of sun) would be difficult. It really likes warm summer sun and warm weather—it is a perfect companion for your tomatoes and peppers. 

Q–How do you control disease on basil foliage?
Basil is usually care-free, but it can get certain diseases.  Prevention is easier than treatment. Good care will help lessen the susceptibility of a basil crop to total destruction by disease.  Choose species and cultivars that are less susceptible. For instance, Italian sweet basil is more susceptible, while, lemon basil, thai basil, spice basil and purple-leafed varieties are less susceptible. 

To grow sweet basil, clean and dispose of any leaf litter from the previous season and rotate garden beds if you can. Grow the basil in full sun with plenty of air circulation.  Mulch plants to prevent splash back onto the leaves and consider using drip irrigation that only waters at the ground level to avoid leaves staying wet for too long too often. Downy mildew and other diseases can come in on infected seeds, so use reputable vendors and treat the seeds in hot water before planting.  When buying a basil plant or pot of seedlings, inspect all the leaves thoroughly. 

If you grow basil in containers, sterilize the containers with a 10% bleach solution and use a sterile, soil-less planting medium.  

If your basil plants do get diseased, for the homeowner, it is probably best to remove diseased plants and plant new plants.   

Basil germinates easily, so growing a backup crop or two is always an option. If the leaves look diseased, pull the plants and start again in a sterile container or new bed. Growing should be fun, so keep moving forward. 

Q–Can you grow tea bush like assam?
Assam tea, like other tea leaves, is made from a variety of the tea camellia, Camellia sinensis. This variety is Camellia sinensis var. assamica.   Black tea, green tea and white tea refer to methods of harvesting and production, rather than denoting different varieties.

We have overwintered Camellia sinensis ‘Large Leaf’ and ‘Lipton Plantation’ for one year, but it was admittedly a mild winter. We have not harvested and processed leaves for tea from our tea varieties. We have not tried assam tea bush, so cannot comment on its hardiness.
Tea is currently grown in South Carolina, Alabama, Washington, Oregon, and Hawaii. Because we in D.C. at Zone 7 are at the edge of its hardiness of Zones 7-9, growing here will likely be experimentation perfect for the curious homeowner.  

Q—Any tips for growing Roselle in containers? Mine is yellowing.
For those who enjoy the herbal tisane of hibiscus tea, you can grow the tropical Hibiscus sabdariffa, or roselle or sorrel.  The bright pinkish red calyxes that remain after flowering can be harvested to make tea or candied hibiscus or a traditional Caribbean drink with ginger, sugar and other spices like cinnamon, cloves or allspice.

Roselle is a tropical plant from India and only survives in Zones 8-11. In D.C. we grow it as an annual, transplanting it into the garden from 4–inch containers started in the greenhouse. If you are growing roselle in a container, make sure to use one that is at least 18 inches deep with good drainage. The root systems will need room to grow, both the short taproots and fibrous roots. Fertilize regularly with a high N, low P, high K fertilizer.  (N-P-K stands for Nitrogen, Phosphorus, and Potassium.  These are the 3 macro-nutrients, or the nutrients that plants need in the largest amounts).

Yellowing can be caused by the wrong amounts of sunlight, water, or nutrients or by cold temperatures. Start seeds indoors and don’t move the container outdoors or transplant outdoors until it is above 40 degrees F. Add a stake if the area will be windy. 

Q–If sage flowers, should you cut off the flowers? What about chive flowers?
When it generally comes to sage, if you are harvesting leaves, try to keep the plant in a vegetative growth state by trimming off buds before they open, are pollinated and produce seed.  This may improve the flavor. You can also start with a new sage plant every few years to keep the leaves and stems tender. However, these practices aren’t necessary, as the flowers are pleasant and attract pollinators to the garden, and the plant can yield small amounts of new tasty growth to harvest for many years even if you don’t trim off the flowers.

Chives generally bloom starting in late April or May in D.C.  Trim the flowers if you want the plant to produce more foliage or if you don’t want it to reseed.  Regardless, the plant should be a perennial for you in a sunny spot for years to come. Again, the flowers attract pollinators and provide their own charm. 

 Q–How do you propagate sage?
Salvia officinalis, or common sage, will start from seed.  It will take a bit of time to get to a size where you can trim or harvest leaves.  Most people start with a greenhouse–grown plant.

Start with a soft wood cutting. This means using the last 2-4 inches of the stem that hasn’t yet become woody.  Cut off half of each leaf to minimize stress and water loss through transpiration.  You want the cutting to focus on growing roots.  Remove lower leaves and place in water or dip in a rooting hormone and press into potting mix or soil-less medium with good drainage.  Mist daily, water thoroughly and allow the medium to drain.

You can also practice root layering by mounding soil around the lower stems of the plant.  You can cover the stems or pin them in place.  This encourages the stems to grow roots that you can make into new plants by trimming off in several months or the next year. Make sure your new plant has leaves, stem and roots. The risk with this method is making the parent plant unhealthy if the soil lies up against the crown or if the soil stays too wet.

Transplant well-rooted cuttings or seedlings into full sun in well–drained soil and water regularly. Some have luck in partial sun.  One of our oldest sage plants gets morning shade and afternoon sun on the west side.  We’ve also had good results with morning sun on the east side. 

Q–How do you modify garden soil when growing Mediterranean varieties?
A lot of soil in the mid-Atlantic region has high amounts of clay.  This has great nutrient-holding capacity, but it tends stay wet and not drain well. Then when it does get dry it takes a lot of water to re-hydrate. In heavy clay soils it is difficult for many plants to get the porosity (air and water space) and ease of growing that they need.

Many Mediterranean plants are used to sandy or gravelly, lean soil. Lean soil doesn’t have too much nitrogen, phosphorus or potassium.  So the great thing is you won’t need much fertilizer or any manure. Unlike many garden plants, these herbs won’t need much organic matter, but adding a little bit of compost, wood chips and bark mulch to help improve the structure of a clay soil is recommended by Curtis Swift, Ph. D. of Colorado State University. He suggests verifying that all of your added organic matter is “coarse” and low in salts (so don’t use most manures).

Swift writes, “An ideal soil is considered to consist of 45% mineral (sand, silt, and clay), 5% organic matter, and 50% pore space.  The pore space provides the oxygen and water necessary for plant growth. Oxygen is necessary for root respiration with research showing the ambient air level (~21% O2) which we breathe is equally beneficial and even critical to plants.”

To amend soil you want to look at improving and preserving its structure and porosity. You can add sand, but it may take matching your clay content 1 to 1 to make much of a difference.  Use course playground sand or horticultural sand rather than anything fine that will fill those precious pores in the soil.  You can also add some gravel of various sizes. Work the soil deeply, trying to double dig or lightly loosen a large planting area. Remove, amend and replace as you go or amend in place.  Use pitchfork tines, stakes, or a shovel to punch holes in the unworked soil below this layer to avoid making a hard-pan where water can gather and water-log plant roots.
You can till to loosen soil, but this risks having the soil become more compacted over time.  Air spaces are critical.  For a Mediterranean herb garden, try leaving in some small stones or adding gravel.  A gravel mulch will help reflect more light onto the leaves and prevent splashing from the soil when it is watered, thereby discouraging disease. Growing in containers, working the soil deeply while preserving structure, and using raised beds are all options for improving drainage. 

What not to do, according to Swift:  Don’t “pulverize” the soil as this turns macropores into micropores and makes water and oxygen less available to the roots. So again, avoid over-tilling. 

Don’t work the soil when wet. 

If you are concerned about nutrients, you can send samples off to a soil lab to determine what nutrients are in your soil.  It is important not to add any nutrient, especially P or K, unless the soil is deficient. Remember, Mediterranean plants like lean soil.  This will also give you your soil pH, which you can determine at home with a kit. It can take a lot of sulphur to lower pH or acidify your soil and a lot of lime (calcium carbonate) to raise the pH or sweeten the soil, so consult your extension service or use formulas for correct application.  The scale runs from 0-14, with 7 being the neutral pH of water. Generally, a pH of 6 to 7 won’t be a problem for Mediterranean herbs. A pH level under 4.5 or above 9 can cause existing soil nutrients to be bound up and unavailable to most plants.   

Generally, I’ve found we haven’t had to worry about pH. Most herbs are pretty forgiving and have a wide range of tolerance. However, if you are gardening in a bed with acidic soil that once perhaps provided a great habitat for pine trees or acidic soil loving plants like blueberries, azaleas or other heath family plants, consider getting the soil tested and adding lime or crushed shells to raise the pH over time or grow your Mediterranean herbs in containers. 

Good luck preparing your bed!  Plant in full sun, leaving the area around the crown clear, add a layer of organic or gravel mulch (which helps protect from frost, heat and disease) and you are good to go.  

Q–Is dill better to grow in the fall?
Dill should be sown into warm soil after frost with soil temps at 60-70 degrees F. The same patch should reseed for the next year if it’s in a warm, protected area.  You can either sow it every few weeks throughout the spring and summer and keep the bed moist or sow in late summer or September for the following year.  If your site is very exposed you may not have success with fall sowing. 

Q–Dill and fennel–don’t grow them near each other because it’s easy to get them confused? Or just label them instead?
Just label your fennel and dill so you know which is which. The idea that they can cross-pollinate to make a new hybrid is not supported by science. They are in the same plant family but in different genera. Inter-generic hybrids are rare.
Cross-pollination is only an issue to worry about if you intend to save and sow seeds of a particular vegetable garden variety that is planted near very closely related plants like certain melons near other melons or squash near other squash. This isn’t something you need to worry about when growing herbs and there is no scientific data that shows dill and fennel crossing.   

Q—How do you propagate herbs from cuttings?
Cuttings are a great way to share your herbs or increase their numbers. Cut soft growing stems with a few leaves, then cut any large leaves in half.  Start the stems rooting in water or in a soil-less medium with a rooting hormone. This should work for sage, thyme and oregano. Some herbs like Anise hyssop (Agastache foeniculum) have seeds that are easy to collect and even start to reseed in place.  Dig seedlings with plenty of soil in the spring.  
Basil will grow from cuttings. Remove a sprig with two sets of leaves and make a clean cut just above the next set of leaves. They really are easy to germinate, however. Gather basil seeds at the end of the season (making sure they are not diseased), let them dry, and store them in the refrigerator until they are ready to be started the following February or March, adding a heat mat underneath until it is warm enough to place them in the sunny windowsill or plant them after frost. 

Q–Is mildew a cosmetic issue? 
I would consider powdery mildew on bee balm (Monarda spp.) to be more of a cosmetic issue.  More and more cultivars are coming out that are more mildew-resistant and less likely to get colonized by the fungus and—if they are—will still grow and thrive and look good. If you are going to ingest the leaves as a garnish or tea, dry or roast them on a pan in the oven on low heat and/or steep them in boiling water. Monarda didyma was used as a tea substitute and Monarda fistulosa can provide a thyme-like flavor since it contains thymol.

Powdery mildew comes in different types; some like wet leaves and some don’t.  Heat and lower humidity during the day and higher humidity at night seems to favor mildews. It gets going in the 70-80 degree F days we have in spring, early summer, and fall, so if you see it in summer, it has been going for a while. 

However, downy mildew on basil is quite destructive and often requires removal. Italian sweet basil is the most susceptible.  Affected basil isn’t table-worthy and the seeds should not be saved.  The garden area should be cleared and a new spot with clean basil seed or plants used the following season. See the question on basil diseases for more detail.  

Q—What are good IPM resources for herbs?  Will fungicide hurt pollinators?  
It’s best to reach out to your local extension agency–often connected to a land grant university—for IPM advice since it will be specific to your area.  You can find your local extension office here: https://www.pickyourown.org/countyextensionagentoffices.htm .  

Q–Where do you order ladybugs?
Search ‘buy beneficial insects’ online to find a range of suppliers.  

Q—Can you overwinter rosemary and lavender?
Both rosemary and lavender will overwinter if you choose hardy varieties and provide them with the right conditions: full sun, correct pH, good drainage, and a layer of mulch. Rosemary likes a slightly acidic pH (6-7.5) while lavender can take a pH of 6.0 but prefers a more neutral 6.5-7.5. They can both withstand a pH as high as 8.5 if their other needs are met. I’ve grown them in the same bed with plenty of hours of either morning or afternoon sun.  Planting by a south-facing wall or in a slightly sloped bed that gets more than 6 hours of all-day sun is best. 

You can try covering the plants with row cover material or a sheet to protect them from extreme freezes. If you are growing a non-hardy variety of lavender, lavandin, or rosemary (anything that can’t survive Zone 7 in D.C. and instead needs to be in the Zones 8-11 range) or prostrate rosemary, which is not hardy to 0 degree F, then count on losing the plant in a bad winter or bring it inside in a pot.  Keeping it outside in a pot actually makes it more susceptible to cold temperatures compared to keeping it in the ground.  You could try piling mulch, soil, or hay around an outdoor pot and covering the plants with cloth but the results may be mixed. 

Q—Is peppermint in pots a good option?
Yes, particularly as this will help the plant not to spread.  

Recipes: 

Pesto  

  • 1 cup fresh basil leaves, chopped 
  • 1-2 cloves garlic, minced 
  • ¼ cup pine nuts, walnuts or sunflower seeds 
  • Salt and pepper to taste 
  • ¼ cup olive oil 
  • ¼ cup parmesan or romano cheese

Mix all ingredients in food processor and serve. 

This pesto can be frozen. Mix all the ingredients except cheese, pack into ice cube trays and freeze. Defrost and mix with cheese to serve. 

Rosemary Focaccia Bread  

Preheat oven to 375 degrees F, 20-30 minutes bake time.

Mix  

  • 2 cups warm water 
  • 2 T. yeast 
  • 2 T. sugar 

Let the yeast interact for a minute or so. 

Add: 

  • 4 T. olive oil 
  • ½ cup vegetable oil (or olive oil) 
  • ½ T. salt 
  • 3 cups flour 

Mix above, then add: 

  • 2 ½ more cups flour 

Work by hand, let rise twice, divide into two 9” round pans.  Top with olive oil, butter, kosher salt, rosemary and oregano.
Bake 20-30 minutes until lightly golden but still pale. 

One Pot Tomato & Basil Linguine 

  • 1 lb linguine 
  • 1 bunch broccoli, chopped into small florets 
  • 1 can diced tomatoes or 4 chopped, seeded tomatoes 
  • 3 T. red wine vinegar 
  • 3 T. olive oil 
  • 3 green onions, chopped 
  • ¼ cup fresh basil, chopped 
  • ¼ cup fresh spinach, chopped 
  • ½ t. salt 
  • 1/8 t. pepper 
  • Parmesan or romano cheese on the side 

 Boil linguini about 5 minutes. 

Add broccoli, boil 2-3 minutes. 

Drain and toss in large bowl or back in the pot with all other ingredients.
Serve with parmesan or romano cheese on the side and garnish with fresh basil. 

Basil Mayonnaise 

  Makes 2 ½ cups   

  • 1 medium bunch basil, 1 cup loosely packed leaves 
  • 1 medium bunch cilantro, 1 cup loosely packed leaves 
  • Juice of 1 lime, 2 ½ tablespoons 
  • 1 teaspoon chili powder 
  • 1 ½ cup mayonnaise (a light olive oil version works well) 
  • ¼ teaspoon salt 

 Place all ingredients in a food processor and blend until herbs are finely minced and incorporated into the mayonnaise. Mayo will store in a clean jar in the refrigerator for a week. 

 Zucchini Fritters 

Makes 20 3-inch fritters 

  • 2 medium zucchini, 1 ½ pounds, 4 cups grated 
  • 1 medium sweet onion, ¾ cup minced 
  • 2 tablespoons minced mint 
  • 1 tablespoon chopped dill 
  • 3 eggs 
  • ½ cup all-purpose flour 
  • 3 ounces feta, crumbled 
  • 6 tablespoons of olive oil  

In a large mixing bowl crack the eggs and beat until the yolk and the egg whites are blended. Add the flour, feta cheese, ground pepper to taste, mint and dill. Stir until barely blended. 

Using a hand-held grater or a food processor, grate the zucchini. Place the grated zucchini on several layers of paper towels and roll up like a burrito. Place the roll in a clean dish towel, and over the sink, twist the ends in opposite directions to squeeze the moisture out of the zucchini. Squeeze until the water is reduced to an ooze. Quite a lot of water will come out, about a cup. Add the zucchini and minced onion to the feta cheese mixture. 

 Heat a large sauté pan to medium heat. Wait until the pan is hot, then add enough oil to just cover the bottom of the pan. Scoop dollops of the zucchini mixture (about three tablespoons worth) and carefully drop into the hot oil. Flatten the dollop slightly. The fritters will resemble small, three-inch nests. Cook until the bottom turns a golden brown, about three minutes. Turn when the edges are browned and the fritters release from the bottom of the pan. Cook until the other side turns golden brown, around two minutes. Place on a platter or cookie sheet covered with paper towels to absorb the extra oil. Add more oil before cooking the second batch.

Repeat above steps until all the batter is gone. 

Serve with sour cream or yogurt.